Nico di Angelo

"With great power... comes great need to take a nap."

- Nico di Angelo

Nico di Angelo is a Greek demigod son of Hades and mortal Maria di Angelo. He is also the younger brother of the late Bianca di Angelo and the half brother of Hazel Levesque. Nico is currently fourteen years old. He is known in the Underworld as the "Ghost King" after taking the title from King Minos.

History
Nico di Angelo was born before World War II, which was before Hades and his brothers took an oath to have no more children. He had an older sister named Bianca di Angelo. Their mother, Maria, died when Zeus struck the hotel they were staying in with lightning in order to kill the siblings. However, they were protected by Hades. Apparently, Hades was not able to protect their mother in time. Both of them were brought to the Lotus Hotel and Casino by the Fury Alecto, who was disguised as a lawyer for their own safety. Alecto was also instructed by Hades to bathe them in the River Lethe to wipe their memories. Some time before being placed in the Lotus Hotel, Bianca and Nico lived in Washington D.C. for a time.

After what the siblings thought was a month in the Lotus Hotel (which was actually approximately 70 years), they were taken out by a lawyer. This lawyer however was a different lawyer than the one who carried them in. Hades released them from the Lotus Hotel because he wanted one of them to be the child of the prophecy and not the "idiot" son of Poseidon, referring to Percy Jackson. They were brought to a military school in Maine, paid for by Hades using what they think is a trust fund left to them by their parents.

The Titan's Curse
Nico first appears with his sister, in a military school called Westover Hall, in Bar Harbor, Maine. Bianca and Nico were almost kidnapped by a manticore named Dr. Thorn. Thalia Grace, Grover Underwood, Percy, and Annabeth Chase attempted to save them, but it is the Hunters of Artemis who come to the rescue. The siblings were sought out by Grover, who believed them to be two very powerful unknown half-bloods. Nico looks to be about ten and seems to be very interested in Mythomagic, a card game about Greek mythology. But when 12 year-old Bianca accepts Artemis' proposal to become a Hunter, Nico gets angry at her for abandoning him. When Bianca goes on a quest to find Annabeth, Percy finds Nico eavesdropping on Bianca and Zoë Nightshade, and Nico successfully convinces Percy to promise to keep his sister safe on the quest. He doesn't appear again until after Percy returned from the quest, where Percy explains to him that Bianca died to save the others on the quest. He immediately turns on Percy, telling him he hates him, and reminding him of the promise he made to keep Bianca safe. Percy tries to comfort him, and Nico claims he can sense her soul being judged. When the last four Skeleton Warriors appear in the camp, Nico tells them to "Go away!" and they are swallowed by a fissure in the ground that Nico creates by accident. Nico is last seen running into the woods at night. Percy then realizes that Nico is a son of Hades after slipping over on and finding the god figurine that Bianca told Percy to give Nico if she died. It was the only one he didn't have (in his Mythomagic collection). The figurine was Hades, Lord of the Dead.

The Battle of the Labyrinth
Nico is first seen through the Iris message that Bianca sent to Percy. He has honed his powers as the son of Hades with help from the ghost King Minos, who had been training him. He tried to summon Bianca's ghost many times, but failed. Frustrated, he seeks to make a deal with his father by trading "a soul for a soul." Percy thinks that Nico is after him, but Nico is after somebody else.

Nico decides to find the soul of someone who cheated death and trade it for Bianca's. Later, he meets Percy, Annabeth, Tyson, and Grover at the Triple G Ranch. He'd entered the ranch to discuss finding the soul he needed with Geryon. Angered, he threatens them, but is stopped by Geryon. He still blames Percy for Bianca's death, and tells him so. He is then tied up by Geryon to be sold to the Titan Army. When Percy offers a deal to Geryon to save him, Nico shouts that he does not want to be saved by Percy.

Percy still saves Nico, and convinces him to try and summon Bianca's spirit again. It works, however, Bianca tells Nico not to be angry at Percy or hold grudges, because that is the fatal flaw of the children of Hades. She says to stop blaming Percy because it is really she that Nico is angry at for abandoning him. She also gives a warning about Kronos sensing Nico, and then she disappears.

He stays at the Triple G Ranch while the others leave. Nico is tricked by Minos into going back into the maze to save Percy, but is captured by two dracanae of Kronos'. In the battle at Daedalus' Workshop, Nico opens a fissure in the ground, sending Minos back to the underworld. He tells Percy that only the dead accept him, and they only do so out of fear.

He escapes with Percy, Annabeth, and Rachel Elizabeth Dare, who have to go back into the Labyrinth to find Grover and Tyson. Along the way, they see Kronos (in the form of Luke Castellan), and Nico, in a desperate attempt to save everyone, shows his true powers, by conjuring a wall of black stone that stops Kronos from following them back in to the Labyrinth but also making it known to Kronos that he is a son of Hades.

When they found Grover and Tyson, they also found the long-lost god Pan, and Nico notices that he is the only one that Pan doesn't talk to.

During the battle at Camp Half Blood, Nico summons undead warriors to fight, and stop some rogue dracanae from reaching the cabins of the gods. The effort nearly kills him and he collapses on the ground at the end of the battle. Percy finds him and helps him by giving him nectar. He releases Daedalus' soul after that, who offers his soul for ransom, but Nico decides Bianca should stay where she is.

He feels out of place at Camp Half-Blood, and refuses to eat with the other demigods or join the singalong at the campfire. Nico summons Bianca's spirit one last time to say goodbye. He tells Percy that he will leave Camp Half-Blood because he feels that he doesn't belong there like his father doesn't belong on Olympus. Before he leaves, Percy gives him the small Hades statue that Bianca had given him before she died.

He appears again at the very end on Percy's birthday at his apartment, and offers a way to defeat Kronos, but is distracted by Percy's blue birthday cake (assuming he's probably never had a birthday party in his life), and Percy offers him some, and they both go inside to talk about Nico's secret plan to beat Kronos.

The Sword of Hades
Nico is one of the three demigods along with Thalia and Percy who are summoned by his step-mother Persephone to find the Sword of Hades. Percy thinks it's a bad idea but Nico is hoping to achieve recognition from his father Hades. He tells Thalia and Percy why they need to go on the quest. To Percy's irritation, he comes up with logical reasons. His knowledge of the Underworld and its occupants helps them complete their quest. He remains in the Underworld afterward to discuss the upcoming Titan War with his father. Before his friends leave, he reminds Percy of his idea to defeat the Titans.

The Last Olympian
Nico's theory to gain an edge over the Titans is to make Percy invincible by having him bathe in the River Styx, similar to the myth of Achilles. It is shown that after Bianca died, Hades feels he has no competent child, repeatedly mentioning that Bianca could do a better job at fulfilling their father's demands.

Nico is first seen in one of Percy's dreams where he is eavesdropping on a group of Titans. It is later confirmed that this was true. Percy sees him again while walking Mrs. O'Leary. Nico is with Juniper and Leneus, discussing Grover's disappearance. Nico, Percy, and Mrs. O'Leary then meet Mrs. Castellan on a disturbing visit by shadow-travel. Soon after, the three enter the Underworld, where Percy is lied and tricked by Nico, trapped by Hades and rescued by Nico. Nico then brings Percy to the River Styx.

It is also revealed that the lawyer who removed the children from the Lotus Hotel and Casino was the Fury, Alecto, in disguise. Nico attempts to summon the ghost of his mother, but Bianca appears and tells him not to. He sees the scene of their mother's death in which Zeus blasted the building she, Hades, and the children were in to pieces. Hades, who realized what was happening too late, only managed to save the children Nico and Bianca. He then tells Alecto to wipe the memories of the children, including their mother's death.

By viewing this scene, Nico finds out who his mother is and is able to convince his father to help the campers in battle. He fights in the Battle of Manhattan with his father, the Lord of the Underworld. Together, they summon wave after wave of undead soldiers against Kronos' forces. He is later seen on Olympus sitting in front of his father when Percy and his friends are receiving gifts from the gods. When Hades pats Nico on the shoulder, Percy says that it's "the happiest he's ever seen him." Nico is last seen ordering undead workers to build the Hades cabin, made up of solid obsidian walls with a skull over the door. There are torches that blaze with burning green fire that will never extinguish.

Between the Series
As stated in The Son of Neptune, in the September following The Battle of Manhattan, Nico rescued the spirit of his half-sister Hazel Levesque from the Fields of Asphodel while looking for Bianca, who had decided to be reborn and thus couldn't be brought back to life. With Thanatos captured, the dead were returning to the world anyway, and Nico felt Hazel deserved another chance at being a hero (Gaea said that he had selfish reasons for doing this, but it is unknown if he had any or for what selfish reasons did he do it for). Hades gives Nico directions on how to find Camp Jupiter, explains why the Olympians have kept both camps separated for centuries, and forbids him of telling anybody about it, as the time isn't right. As stated by Dakota, Nico took Hazel directly to Camp Jupiter and demanded Reyna to take her in.

The Lost Hero
Nico is mentioned by Annabeth to be searching for Percy along with Tyson, Grover, and the Hunters of Artemis, but otherwise doesn't appear. It's to be assumed that he doesn't stay at Camp Half-Blood much, as Annabeth said he "sometimes" shows up, which could be due to his double duty at Camp Jupiter as Ambassador of Pluto.

The Son of Neptune
Nico meets Percy at Camp Jupiter, where he is visiting his half-sister, Hazel Levesque. When they meet, Nico is shocked but easily regains control over it. He poses as a son and ambassador of Pluto, and even though most of the campers see him as harmless, Hazel knows that Nico is more dangerous than Reyna, Octavian, and maybe even Jason Grace. Nico later tells Hazel, after Percy leaves with Frank Zhang, that he knows Percy, but Percy must regain his memories by himself, since Percy needs to find his own way at the camp and can't interfere. It is later revealed that Nico was forbidden to do so by Hades, since the time wasn't right yet.

Later, Nico is shown to be observing the War Games using a pair of binoculars. When Mars appears, Mars glares at Nico while mentioning that people will no longer be held to death, implying that he knows about him saving Hazel's spirit. Nico is later shown in the meeting, where he is present as a Senator, and the Ambassador of Pluto. He supports Frank, Percy and Hazel by telling them about the first Gigantomachy, Alcyoneus, and Thanatos, all of which makes Reyna suspicious. However, Nico doesn't say anything when Octavian protests and wins the argument. Nico leaves Camp Jupiter before Hazel to see if he can locate the Doors of Death from inside the Underworld.

Nico is later shown in one of Hazel's blackouts. He finds Hazel in the Fields of Asphodel and brings her back to the mortal world to give her another chance at life.

During his search for the Doors of Death, Nico (believing he could go anywhere in the Underworld) comes too close to Tartarus and gets pulled in by the dark gravity the pit generates (similar to a black hole). During the quest for the god Thanatos, Gaea mentions that when she used Hazel, she was at least honest about it, unlike Nico who she claims was lying to Hazel. Gaea also says that Hazel should be happy that she captured him. She later tells this to Percy and he promises that they will find out what happened to him.

When they free Thanatos, Hazel asks if Nico is alive. While Thanatos doesn't say if he is alive or dead, he does say they will find the answers in Rome. Later after Percy regains his memory, he calls Mrs. O'Leary, much to Hazel's surprise. He tells her that Nico could tell her the story before realizing what he had said. On the way back to Camp Jupiter, Percy fully regains his memory, and is quite angry at Nico for not telling him about his past. After Polybotes is defeated, Hazel mentions that Pluto is turning a blind eye to her to let her live and she feels that he wants her to find Nico.

The Mark of Athena
Even though Nico is a son of Hades, he barely survived the trip into Tartarus, and almost went insane. Gaea's forces went down there and overwhelmed him almost instantly, since they are extremely powerful in Tartarus. Afterwards, in order to lure the seven heroes into a trap, Gaea had Nico transported out of Tartarus through the House of Hades (an underground temple in Greece), and taken to Rome, where her twin giant sons Otis and Ephialtes sealed him in a bronze jar, similar to how Ares was once held captive by the two giants.

Percy sees him in his dream, imprisoned in the jar in a coma-like state. He had marked three scratches on the wall of the jar, one for each pomegranate seed he had eaten. The seeds are a last resort food, which only children of Hades or Pluto can eat. Hazel explains that the seeds last for up to one day and put him in a death like state so that he doesn't use up all his air, something Hazel calls a death trance. Percy, Jason and Piper go to Nico's rescue, which they know is a trap set up by the earth goddess Gaea. Prior to discovering the twin giants Ephialtes and Otis, who were holding Nico captive, the trio came across an abandoned nymphaeum, whose nymphs they set free.

After making it to Rome, Hazel, Frank, and Leo try to find Nico, which leads them to the Pantheon, but they are lead into a trap, and are almost killed by eidolons. After defeating them, Leo realizes that Hazel was trying to track Nico by using his sword as a beacon, which they find lying in the workshop.

Meanwhile, during Percy and Jason's battle against Otis and Ephialtes, the jar Nico was held in breaks open and Piper manages to pull the incredibly weakened Nico to safety. Later back on the ship, Nico tells them how he was captured and what he now knows about the Doors of Death. Nico had found out that the Doors of Death have two sides: one in the mortal world and one in Tartarus. The mortal side of the portal is in Greece, in a place called the House of Hades and it is guarded heavily by Gaea's forces. Nico then says that the other side of the doors are in Tartarus, a place not even the gods go and that is why Hades has not been able to close the doors. Not even the god of death, Thanatos, would go near Tartarus.

Leo guesses that the seven heroes will have to go to Tartarus and close that side of the portal, but Nico says that it would be impossible, not even they would stand a chance down there. Percy then says that they can set sail for the House of Hades, defeat Gaea's forces, and close the doors on the mortal side. Nico however says that it is not that easy and that the Doors of Death have to be controlled on both sides to be closed, as they are like a double seal. He says that maybe all eight of them working together could defeat Gaea's forces on the mortal side, but that they would need a team fighting simultaneously on the Tartarus side; a team powerful enough to defeat a legion of monsters in their home territory.

In Arachne's lair, when Percy tries to save Annabeth from falling into Tartarus, Percy catches hold of a ledge just in time. Nico tries to help, but he is out of reach. Annabeth tells Percy to let go, but Percy says that he will not lose her again. Percy tells Nico that they will see the rest of the seven and Nico on the other side, and makes him promise to lead the others there. Nico promises and Percy lets go, allowing the dark gravity of Tartarus to pull him and Annabeth in. During a meeting on the Argo II, Nico admits that Percy is the most powerful demigod he has ever met and with Annabeth there with him, he is sure they will be safe. He assures Leo that both Percy and Annabeth are still alive.

The House of Hades
The Argo II is being attacked by earth spirits who are attacking with boulders. Nico is standing on the foremast when it is hit by a boulder and collapses. Hazel Levesque runs to help Nico up from the debris. Soon afterwards Hazel tells Leo that she sees Arion running towards them and that they should land to meet him. Leo replies that they should stay away from landing on the ground, but he will get Hazel as low as possible. Nico asks who Arion is and is told by Leo that he is Hazel's horse.

Personality
In The Titan's Curse, Nico is ten years old and enjoys playing with his Mythomagic cards that none of the others truly seem to understand. He is totally clueless and very inquisitive: he is curious and questioning and likes Percy a lot like a big brother, and begins annoying Percy with his ceaseless questions, especially when he asks if Annabeth Chase was his girlfriend, which makes Percy consider "stuffing him in a meat flavored sack and throwing him to the wolves." He is excited about finding out that he is a demigod (immediate response: "Cool!"), and compares monsters to Mythomagic creatures. He feels abandoned when his sister Bianca, who practically acts as his parent, becomes a Hunter of Artemis. When she goes on a quest, Nico makes Percy promise to protect her. After her death, he develops a long-lasting grudge toward Percy for letting her die. Once he realizes that he is a son of Hades, he casts away his childhood interests and runs off on his own.

In The Battle of the Labyrinth, he becomes grim and solitary and definitely a more powerful character, saying that "only the dead respect me...and only out of fear." His attitude change may have happened partially because of Bianca's death and partially because of Minos' manipulations. He believes that he will never be accepted by the living and does not belong at Camp Half-Blood in comparison of his father feeling out of place on Olympus. He is also shown to be somewhat vengeful, as he wanted revenge for Bianca's death, something he blamed Percy for. However, Nico forgives Percy after Bianca's ghost tells him that it wasn't his fault that she died. He also accepts the small figurine of Hades that Bianca wanted him to have.

In the same book, he is shown to be very naive and gullible, being able to believe that Minos was actually helping him bring Bianca back from the dead. Nico cares much about his sister and the constant attempts to bring her back show that he would not give up without trying. Nico is shown to be very loyal to the Olympians, as he could have easily joined Kronos and the Titans, but chose to fight on the Olympians' side.

In The Last Olympian, he is quite sincere in wanting to help Percy with the prophecy and the war. However, Nico can be somewhat untrustworthy at times. This is shown when he tricked Percy into coming to the Underworld so he could gain answers about his mother, but Nico wasn't planning on Percy becoming imprisoned. By standing up to both Kronos and Hades, he is shown to be that he is very brave and bold. He likes being on his own, not really wanting help from anyone. He keeps to himself a lot and is pretty secretive, which causes plenty of problems throughout the series. Nico wants to help Percy, and was very miserable at the thought that Percy did not trust him after his dad tricked him as well, by the end they were back to a brotherly relationship. Nico is later proven wrong in the belief he will not be accepted by the living. After the war is over, all the campers of Camp Half-Blood welcome Nico cheerfully: as Percy puts it, "Hey, show up with an army of undead warriors to save the day, and suddenly you're everyone's best friend." This helps lift Hades's curse from the Oracle.

In spite of Nico pretending not to know Percy in The Son of Neptune, he spoke very highly of him to Hazel, telling her that he is dangerous to his enemies and a valuable ally to his friends. Nico's relationship with Percy also shows how much he's grown and matured throughout the series. Nico is very caring to Hazel Levesque, his half sister, and tries to help her as much as he can. His helping her sort of parallels with how Percy helped him in several cases. This shows that he loves his family, even his half-siblings. Unfortunately, after returning from Tartarus in The Mark of Athena, Nico is very different. He turns more inward and withdrawn. His eyes became somewhat glassy and Percy wonders if something in him broke permanently.

Nico can be extremely dangerous as he is a son of the Big Three. He is very unpredictable, and due to his sister's death can be somewhat mentally unstable as demostrated in The Battle of the Labyrinth. He has a short temper, just like Percy and Thalia.

Fatal Flaw
"Holding grudges has always been the fatal flaw of children of Hades."

- Bianca to Nico

Nico di Angelo's fatal flaw is holding grudges, as Bianca's ghost tells him. In his case, it can lead to not wanting help or not asking to be saved in deadly situations because he doesn't like someone. Examples are his not wanting to be helped by Percy, even when Geryon was going to sell him to the Titans and not wanting to go back in the Labyrinth with them. Bianca shares this with him, as she also had the same fatal flaw in life, although this didn't lead to her death. Bianca also says that holding grudges is the fatal flaw of Hades' demigod children. In the fifth book, Nico tells his father, Hades, that he is holding a grudge that could end up killing the Olympians and the campers. Ever since The Last Olympian, Nico hasn't demonstrated this flaw any more, implying that he has learned from his previous experiences, and tends to avoid holding grudges.

Appearance
He is said to have black hair that always looks like he just got out of bed (messy and shaggy), olive skin that has paled to white over the course of The Battle of the Labyrinth, dark eyes, and wears mostly black clothing. It was stated by Percy that Nico has his mother's eyes, but it has also been said that he has the wild glint of a madman in them, just like his father.

By the end of The Battle of the Labyrinth, he has a silver skull ring which is said to belong to his father (as stated by Hazel), and he has a Stygian Iron sword always by his side. He is also mentioned on several occasions wearing an aviator's jacket, most likely because he was originally from WWII. He wears a black trench coat when he attempts to bring back Bianca, as shown in one of Hazel's blackouts. Hazel also points out how his fingers are ghostly white, like their father. In The Son of Neptune, Nico is seen wearing a black toga when Percy goes to talk to the senators (the reason being that togas are only worn on special occasions, like meetings). Nico always wears his Stygian Iron sword on his side in his chain belt. In  The Mark of Athena, Nico's eyes are described as "looking like shattered glass", due to his ineffable experience in Tartarus.

Some time after escaping Tartarus, Nico is still described as being very thin, with his black shirt and jeans hang loosely from his body. His dark hair is longer and frames his face. However, the area around his eyes is very dark, as well as his olive complexion becoming much paler, almost the color of tree sap.

Abilities

 * ADHD: Like all demigods, Nico possesses inborn supernatural battle reflexes and senses that he uses to analyze the fighting style of his opponent.


 * Dyslexia: Nico's brain is "hard-wired" for divine Ancient Greek instead of modern languages.


 * Fighting skills: Initially, Nico shows more skill with his powers than his fighting skills. However, this might have been due to his lack of formal combat training at camp. In The Demigod Files, he did show decent combat skills shown he was able to keep up with Percy and Thalia when they fought a group of Keres. Not only did Nico emerge unscathed from the battle, but he easily defeated the Keres he fought. Later, he even fought the Titan Iapetus alongside Percy and Thalia (but he was quickly incapacitated). In The Last Olympian, Nico fights in the Battle of Manhattan, and emerges unscathed, implying excellent fighting skills. In The Son of Neptune, Hazel mentions being impressed the first time she saw him fight, suggesting that he honed his skills between the two series, possibly during his stays at either Camp Half-Blood or Camp Jupiter. Nico was even able to fight Gaea's army in Tartarus itself, though he was overwhelmed by their sheer numbers.

Demigod Abilities
As a son of Hades, Nico is an extremely powerful demigod. He is considerably more trained in using his powers than most children of the Big Three, such as Thalia and Percy, due to his time spent in the Underworld and he states that he trains with the dead. In The Son of Neptune, Hazel stated Nico was the most powerful demigod she'd ever seen, an impressive admiration given that she knew Jason Grace and dozens of other demigods (though meeting Percy caused her to question this judgment). Nico himself states that Percy is the strongest demigod he has ever met.
 * Geokinesis: He can control the earth - levitating rocks and making them deadly, black, and extremely sharp. Nico can open fissures in the earth that can swallow up creatures and trap them in the Underworld. He can cause earth tremors so powerful, they can shatter the pillars of a mountain fortress. He can also control the black walls of the Underworld. His geokinetic ability is only surpassed by his half-sister, Hazel Levesque.
 * Necromancy: Nico can summon the dead, put the dead to sleep, and communicate with the dead. He also has divine authority and control over the Furies and the dead. He can banish the dead and release people who should have died (i.e. Daedalus). Some of his abilities in this domain includes the power to destroy Skeleton Warriors and physically manipulating ghosts, such as taking hold of them by hand or forcing them to be silent with a gesture. In The Last Olympian, he was able to summon a fairly large army of undead soldiers by himself. It is unknown how much he has increased in power since then. Hazel once stated that she and Nico each controls an aspect of Hades/Pluto's powers: death (for Nico) and riches (for Hazel).
 * Sensitivity to Death: He can sense death as it happens or when it is coming; when a person close to him dies, it comes as a ringing in the ears. He can also sense when an immortal is fading, as on the case of Pan.
 * Life Aura: He can sense someone's life aura (he sensed it when Rachel Elizabeth Dare's life aura flickered, and he told her, "I could see you dying.") In addition to that, he has a deep knowledge of the Underworld and knows when a person's life is being judged in the Underworld.
 * Umbrakinesis: He has control over darkness and shadow for various uses such as solidifying it for attacks and to hide in it.
 * Shadow Travel: He can use shadows as a way of transportation, however, great distances tend to wear him out. The first time he shadow traveled, he passed out for a week. He has said that while learning to use it, he ran into walls and made accidental trips to China.
 * Induced Fear: He radiates fear and death like his father, Hades, but to a much lesser degree.
 * Death Trance: Nico can also go into a hibernation called a Death Trance using the seeds of Persephone. It induces him to essentially die for a day so he doesn't consume oxygen.

Percy Jackson
A young Nico had the ability to annoy Percy quite easily, as seen in The Titan's Curse when he asked Percy several questions about his relationship with Annabeth. Near the end of the book however, Nico begins to blame Percy for his sister's death, making him much more cynical. Percy, true to character, feels partially at fault and responsible for Nico.

In The Battle of the Labyrinth, Percy receives Iris Messages from Bianca regarding Nico. The two later meet at Geryon's ranch, where Percy makes a bet with Geryon. Nico is included, which angers him, as he shouts that he doesn't want any favors from Percy. After Percy defeats Geryon, he and Nico talk to Bianca, who tells Nico to stop holding a grudge against Percy when it is she he is angry with. By the end of the book, Nico has forgiven Percy. In the last chapter, he appears and informs Percy of a plan he has.

In The Last Olympian, Nico leads Percy to his father's palace in order to gain information on his mother, not knowing that Hades plans to lock Percy up. Percy feels betrayed, even when Nico eventually tries to help him. After Percy bathes in the Styx, he tells Nico to stay and convince Hades to join the Olympians in battle. Nico appears glum, stating that Percy can't trust him anymore. The two join to battle in final battle and Percy forgives Nico.

After Percy has lost his memory in The Son of Neptune, Nico pretends to not know him, and it's later revealed in Mark of Athena that Hades had told him the time was not right for Percy to know. Once Percy regains his memory, he feels the urge to strangle Nico for not telling him.

In The Mark of Athena, in Percy's narration, it is mentioned that Percy had wanted to strangle Nico about what he did, but doesn't, due to how Nico is after being kidnapped. Nico states (after Annabeth and Percy's fall) that Percy is the most powerful demigod that he knows. During Nico's kidnapping, Percy doesn't give up on finding him, showing that he cares a lot about him. Percy can tell that Nico may - or may not - have seen something in Tartarus that shattered him permanantely, which shows that they know each other well, despite their previous situations.

Nico and Percy share a brother-like relationship and Percy feels responsible for Nico's safety. Nico feels respect for Percy, stating that he is dangerous to his enemies and a good person to have beside you in battle.

Annabeth Chase
While their friendship is not mentioned extensively, Nico respects Annabeth and is at times shown to listen to her more than Percy, possibly to annoy him. Annabeth later suspects that Nico had a crush on her which was hinted although she has no feelings beyond kinship.

Frank Zhang
There is not much known about their relationship. While Nico is kind of mysterious and makes everybody else nervous, Frank doesn't feel the same way and may trust him because of Hazel. It is also stated that Nico finds Frank amusing.

Thalia Grace
There is not much known about their relationship. In The Sword of Hades, Thalia calls him Bianca's little brother. Nico doesn't like being called that, but he seems to be on good terms with her.

Hazel Levesque
In The Son of Neptune, Nico is shown to be very caring to Hazel Levesque, his half sister, and tries to help her as much as he can. Nico occasionally mistakes her for Bianca, which often reminds Hazel that she isn't the one he was looking for when he entered the Fields of Asphodel. Despite this, the two care deeply for each other. Nico is one of the few people who know about Hazel's past and was the one who helped her during her blackouts. The two represent the Greek and Roman form of Hades/Pluto as in Greece, Hades was known for death and in Rome, Pluto was known for his wealth. Hazel remarks that before she met Percy, Nico and Jason were the two strongest demigods she had met.

Near the end of the book, Gaea informs Hazel that she has taken Nico and that he should not have gone looking for the Doors of Death.

Bianca di Angelo
Nico and Bianca shared a very close relationship. As their mother died when they were young, Bianca assumes an almost motherly role, bearing the responsibility that a parent would normally bear, shown when Nico stated Bianca is strict on the movies she lets him watch, before he watched the Orientation Film. This is one of the reasons why Bianca joined the Hunters of Artemis. In The Titan's Curse, Bianca tells Percy that she wanted to know what it would be like to "not be a big sister for one day." Nico felt betrayed when this happened and managed to convince Percy to follow her and keep her safe for him. When Percy failed to do so, Nico was angered and claimed that he could feel her soul being judged, causing Percy to realize he is a son of Hades.

In The Battle of the Labyrinth, Nico is shown to be attempting to bring Bianca back to life through the exchange of a soul for another soul. In search for this soul, Nico meets Percy at Geryon's ranch. Percy tells him that Bianca has been sending him Iris Messages, which angers Nico, as she refuses to see him. Later, Bianca appears. She tells Nico to stop holding a grudge against Percy and it is she that Nico is angry at, not Percy. Later in the book, Nico is seen talking to Bianca again.

She appears once in The Last Olympian, attempting to dissuade Nico from summoning their mother. Her warning goes unheeded.

In The Son of Neptune, it is mentioned that Nico goes searching for Bianca when Thanatos is kidnapped but finds that she has been given rebirth. Instead, he finds Hazel and brings her back to life.

Maria di Angelo
Nico has no memories of his mother, which is why he is so eager to discover more information from Hades, who uses this to his advantage at times. He attempts to summon her, despite Bianca warning him that he should not.

In his attempt to summon his mother, he is presented a memory which discusses the terms of the agreement after World War II and how Hades defied it. Nico watches as Hades and Maria discuss how Hades can bring her to safety, though his offer of building her a palace is declined. He then sees Zeus destroy a hotel in an attempt to get rid of him and Bianca.

Hades
Nico and his father share a complicated relationship. They are both extremely similar, especially in their ability to hold grudges. Both of them are also somewhat cynical. However, Nico seems to have a stronger tie to his friends, while Hades seems to have a much darker view of his siblings. Nico is also a more genuine person than his father, Hades can be crafty, as seen from how he used Nico to attempt to trap Percy.

Hades seems to care a great deal for his son and want to redeem himself in the Olympians' eyes through Nico. Nico also appears to want to please his father. At the same time, Nico is unafraid to challenge his father in order to do what Nico believes is right. Hades occasionally remarks that it would've been better if Bianca survived, much to Nico's dismay, but in the end, he views his son with pride.

Persephone
Nico and Persephone share a mutual disliking of each other as he is a reminder of Hades' affair with Maria di Angelo, who Persephone is jealous of. This same dislike appears between Percy and Amphitrite, as well as Thalia and Hera. Persephone appears to take it to another level through turning Nico into a dandelion as stated by him in "The Demigod Files" when he, Percy and Thalia ended up in the underworld together.

In The Mark of Athena it is revealed that Nico carries around some pomegranate seeds from Persephone's garden in case of an emergency, and so uses them when he is trapped by the twin giants. It is unknown how Persephone feels about Nico using an item from her personal garden, or if she is aware at all.

Camp Jupiter
To everyone at Camp Jupiter, Nico is just a traveling outcast. However, Reyna and Octavian grow suspicious when Nico seems to know about Alcyoneus in the meeting. Camp Jupiter proclaims him as the Ambassador for Pluto.

Items

 * A Stygian iron sword. It is three feet long, and in The Last Olympian is described as being "black as a nightmare." It helps Nico to channel his power to control and summon the dead, and it can also drain souls to give the sword more power. Only Hades and his children can use Stygian Iron for their weapons.
 * He is seen wearing black armor in The Last Olympian with a skull shaped visor.
 * A skull ring shown in The Battle of the Labyrinth, The Last Olympian, and The Son of Neptune. Twisting it makes the nearest dead restless. He usually uses it to relieve tension.
 * Mythomagic figurines and cards. He throws them into a fire in anger during The Battle of the Labyrinth.
 * In The Battle of the Labyrinth, he had a pair of Daedalus' wings, but after he used them, he threw it away in a trash can.
 * A red travel rose that he didn't use, given to him by Persephone, in The Sword of Hades.
 * Pomegranate Seeds-magical seeds from the Underworld. If he places them on his tongue, he can go into a death like trance that allows him to use less air and energy, in the event he is captured. This allows him to go into what is called a "Death Trance."

Etymology

 * The name Nico is the diminutive form of Nicolò or Nicola, the Italian form of the Greek Nikolaos (Nicholas), meaning "victory of the people." It is most likely intended to evoke the prefix necro-, meaning dead. His surname is Italian for "of the angels" alluding to his role as the Lord of the Dead's son.
 * Nico is also a form of Nike, goddess of victory.

Trivia

 * The line "With great power...comes great need to take a nap." is a near obvious reference to one of Spider-Man's most famous line: "With great power, comes great responsibility", albeit a bit altered for comedic effect and to indicate that Nico's powers tire him greatly, prompting him to take naps every now and again to regain his strength, as he did at that moment in The Last Olympian.
 * Nico's physical description (pale skin, dark hair, brown eyes, black clothes, and an aura of death) also fits Rick Riordan's description of Anubis, the Egyptian god of funerals, in The Kane Chronicles series.
 * The first time he met Mr D., he called him "The wine dude", which irritated the god, but he forgot about it soon after.
 * Nico was turned into a dandelion for a short period of time by Persephone during a "family spat." This was never seen directly in the books, but is talked about by Nico and Persephone in The Demigod Files.
 * Nico has been to a New Orleans graveyard, though it is unknown why he went. Interestingly, this is one of the favorite hangouts of another death god (Anubis, the Egyptian god of funerals). It was also the place that his half-sister Hazel Levesque lived just before her death.
 * Nico and Jason Grace have similarities:
 * They both have older sisters, whom have both been/are Hunters of Artemis.
 * They have both been to Camp Half-Blood and Camp Jupiter.
 * They have godly stepmothers (Persephone, Juno).
 * Both have lost their memory. Nico's because of being dipped in the River Lethe and Jason's was removed by Hera/Juno (though, Jason's is temporary).
 * Hazel Levesque and Nico's mothers have very similar first names: Marie Levesque and Maria di Angelo.
 * Nico's birthday is undecided. Rick Riordan has stated on two different occasions his birth-date: January 28th, 1924 and May 16th, 1924. However, the year would be way off since in The Son of Neptune, Hazel stated he was only a few years younger than her.
 * In the series, he is described as having long, shaggy black hair, but in his portrait he has short brown hair. Also, he seems to have green eyes instead of brown, and not as pale of skin.
 * He is the only child of the Big Three to never have gone on a quest. However, he, Percy, and Thalia were given the task of retrieving the then unfinished sword of Hades, and succeeded.
 * Unlike his half sister Hazel, horses don't like Nico ( as Porkpie, one of Blackjack's friend once stated: "He smells like the dead.")
 * As of The Mark of Athena, Nico is the only known demigod ever to have returned from Tartarus alive but even as a son of Hades, his stay in Tartarus left him shattered and more withdrawn.
 * Nico has the ability to control ghosts. However Melinoe is the goddess of ghosts, not Hades.
 * He was the first character in both series to be shown playing the game Mythomagic and collecting Mythomagic figurines.