The Heroes of Olympus is Rick Riordan's sequel series to the Percy Jackson and the Olympians series. It focuses on Greek-Roman mythology instead of only Greek mythology like its predecessor series. The sequel series to it is The Trials of Apollo.
"Seven half-bloods shall answer the call,
To storm or fire, the world must fall.
An oath to keep with a final breath,
And foes bear arms to the Doors of Death!"
- Jason Grace- Jason Grace is the son of Jupiter and Beryl Grace, brother of Thalia Grace and one of the seven heroes. He went to Camp Jupiter and was Praetor until his memory was wiped by Hera and he was sent to Camp Half-Blood. He is the brother of Thalia Grace and the boyfriend of Piper McLean. He can fly, control lightning, control and summon venti and create different types of storms (thunderstorms, rainstorms, and hurricanes). He has a horse named Tempest. He is best friends with Leo Valdez. At the end of the series, Jason decides to stay at Camp Half Blood and leave Camp Jupiter. He is close friends with Nico, Frank and Percy. Jason is also the main love interest for both Piper and Reyna.
- Piper McLean- Piper is the daughter of Aphrodite and movie star Tristan McLean, born in Tahlequah and lived in Malibu. She is one of the seven heroes. She ended up being sent to Camp Half Blood and claimed by her mother. Piper has the ability to Charmspeak and speak French, and has a dagger, Katoptris. She is the girlfriend of Jason Grace and the best friend of Leo Valdez, Hazel, and Annabeth. She is Aphrodite's cabin’s head counselor. She is close friends with Reyna and Frank.
- Leo Valdez- Leo is the son of Esperanza Valdez and Hephaestus, and one of the seven heroes. Leo was in and out of foster homes his entire life until he was claimed and sent to Camp Half Blood. He was later accidentally sent to Ogygia and fell in love with Calypso, and he later rescued her. Leo is the first son of Hephaestus to have fire powers in over three hundred years and helped build the Argo II. He has a pet robotic dragon named Festus and is the best friend of Piper McLean and Jason Grace. He is close friends with Hazel and Frank.
- Percy Jackson- Percy is one of the seven heroes and the son of Poseidon and Sally Jackson. Percy lived at Camp Half-Blood until his memory was wiped by Hera and he found Camp Jupiter and was trained by Lupa. Percy befriended Frank and Hazel and went to Alaska, and later joined the Argo II and fell into Tartarus (place) with Annabeth. Percy has water powers and can generate hurricanes like Jason. He is best friends with Hazel, Frank, Jason, Nico, Thalia, and Grover, and is the boyfriend of Annabeth Chase. He has a sword named Anaklusmos, also known as Riptide. He was briefly praetor.
- Annabeth Chase- Annabeth is one of the seven heroes and the daughter of Athena and Frederick Chase. Annabeth ran away from home at the age of seven and found Camp Half Blood. Annabeth helped Leo, Piper, and Jason build the Argo II and helped find Percy at Camp Jupiter. She also found the Athena Parthenos and fell into Tartarus with Percy. Annabeth had a celestial bronze knife and a laptop from Daedalus that she lost in Tartarus. Her weapon is now a drakon-bone-sword. She is the girlfriend of Percy Jackson. She is best friends with Thalia, Grover, and Piper, and close friends with Frank and Hazel.
- Hazel Levesque- Hazel is one of the seven heroes and the daughter of Marie Levesque and Pluto. Hazel died in 1942 trying to stop Alcyoneus from rising and was in the Fields of Asphodel for years, until Nico rescued her and brought her to Camp Jupiter. She later befriended Percy and joined the heroes on the Argo II. She can control riches, sense death, and learned how to control the Mist. Her weapon is a spatha and later became Centurion of the Fifth Cohort. She is the girlfriend of Frank Zhang. She is best friends with Piper and Percy, and close friends with Leo and Annabeth. She is the half sister of Nico.
- Frank Zhang- Frank is one of the seven heroes and is the son of Emily Zhang and Mars, and a legacy of Poseidon. After his mom died in Afghanistan, his grandmother sent him to Camp Jupiter and joined the fifth cohort. Frank went on a quest to Alaska with Hazel and Percy and later joined the Argo II. Frank is a skilled archer and has the ability to shapeshift into animals, but his life is tied to a piece of wood. He is the boyfriend of Hazel Levesque and distant relative of Percy. Frank was promoted to praetor by Jason. He is a co praetor with Reyna. He is close friends with Jason, Annabeth, Leo, and Nico.
- Nico di Angelo- Nico is the son of Maria di Angelo and Hades, the brother of Bianca di Angelo and the half brother of Hazel Levesque. He was born in the 1930s and lived in the Lotus Hotel, not aging for years. Nico is not one of the seven heroes, but went to Tartarus and was rescued by the seven after he was imprisoned by Otis and Ephialtes. He then helped the seven open the doors of death at Epirus and deliver the Athena Parthenos to Camp Half-Blood. Nico can summon and control the dead, shadow travel, control the earth, turn living beings in to ghosts, spirits, or just kill them by just looking at them and has a Stygian iron sword. He is a close friend of Jason, Percy, Frank, and Reyna. He had a crush on Percy. He is dating Will.
- Reyna Ramírez-Arellano- Reyna is the daughter of Bellona and Julian Ramírez-Arellano, and the sister of Hylla. Reyna was born in San Juan and later lived on Circe’s island, until it was destroyed, and she went to Camp Jupiter and became praetor. Reyna is not one of the seven, but she flew to the ship across the Atlantic on her Pegasus and later helped deliver the Athena Parthenos to Camp Half-Blood. Reyna has a dagger and a cloak, and has the ability to share her strength with others. She is close friends with Annabeth, Piper, and Nico. Reyna's main love interest is Jason Grace, but she believed he died after his 8 month disappearance and briefly considered Percy a romantic partner. She was a co praetor with Frank.
- Gleeson Hedge- Coach Hedge is a satyr at Camp Half-Blood, a former protector of Piper, Leo, and Jason, and escorted Clarisse to Camp. Hedge later accompanied the seven heroes to Greece on the Argo II, and helped Reyna and Nico escort the Athena Parthenos to Camp. He is a mentor to all of the seven but mainly Piper, Leo, Jason, Nico, and Reyna. He is the husband of Mellie and the father of Chuck Hedge.
- Hylla Ramírez-Arellano- Hylla is the daughter of Bellona, sister of Reyna, and queen of the Amazons. She lived on Circe’s island with Reyna until it was destroyed, and she joined the Amazons. Hylla helped Camp Jupiter during the attack of Polybotes and helped the Hunters of Artemis defeat Orion. She overthrew Otrera and is friends with Hazel.
- Thalia Grace- Thalia is the daughter of Zeus and sister of Jason Grace. Thalia sacrificed herself to save Luke and Annabeth and turned into a tree, but she was revived. She is currently the lieutenant of Artemis and is a member of the Hunters of Artemis. She helped defeat Orion, Khione, Lycaon, and Porphyrion. She is best friends with Grover, Percy, and Annabeth.
- Tristan McLean- Tristan McLean is a Cherokee movie star and the father of Piper, born in Tahlequah. He was kidnapped by Enceladus and was saved by Piper, Jason’s, and Leo, but they forced to wipe his memory after it, so he forgets that Piper is a demigod. He lets Piper stay at Camp and Mellie becomes his assistant.
- Octavian- Octavian was the Centurion of the First Cohort at Camp Jupiter and an Augur. He is a legacy of Apollo. Octavian did not trust the Greeks and later rebelled against Reyna and waged war against the Greeks. He is rivals with Reyna, Jason, Percy, Frank, Annabeth, Hazel, Leo, Piper, Nico, and Will.
- Dakota- Dakota is a Centurion of the fifth cohort at Camp Jupiter and the son of Bacchus. Dakota stood by a Hazel when she arrived at Camp and later was saved by her. He later helped Nico and Reyna sabotage and stop Octavian and Michael.
- Michael Kahale- Michael is the son of Venus and centurion of the First Cohort. Michael was Octavian's right hand man and was loyal to him, helping him try destroying the Greeks. He tried to arrest Nico and Reyna for treason. He is from Hilo, Hawaii.
- Will Solace- Will is a healer and a son of Apollo, and the head counselor of Apollo’s cabin at Camp Half-Blood. Will helped Nico, Lou, and Cecil defeat Octavian and save camp. He is the boyfriend of Nico. He is also a skilled archer and a prominent healer at camp half-blood.
- Drew Tanaka- Drew is the daughter of Aphrodite and the former head counselor of her cabin. She is very rude and mean and manipulated her cabin-mates. She was overthrown by Piper, her sister. She had a crush on Jason.
- Cecil Markowitz- Cecil is the son of Hermes and lives at Camp Half-Blood. Cecil helped defeat Octavian and sabataged his onagers.
- Lou Ellen Blackstone- Lou is the daughter of Hecate and the head counselor of Hecate’s cabin. Lou later helped defeat Octavian and manipulated the mist, and used a Pig Ball to turn Romans into pigs.
- Rachel Elizabeth Dare- Rachel is a mortal who can see through the mist and the Oracle of Delphi. She said the Prophecy of Seven and helped Leo, Piper, Jason, and Annabeth. Rachel later got a message from Annabeth in Tartarus and gave it to Reyna, trying to get her to help them.
- Chiron- Chiron is a Centaur at Camp Half Blood and mentor to the students. Chiron told the demigods about the existence of Roman demigods and helped find Percy.
- Mellie- Mellie is a cloud nymph, the wife of Coach Hedge, and the mother of Chuck Hedge. She was the former assistant of Aeolus and was rescued by Leo, Coach Hedge, Jason, and Piper. She later saved the four from Aeolus and became the assistant for Tristan.
- Calypso- Calypso is the girlfriend of Leo. The two fell in love when Leo fell on Ogygia. Calypso did not like him at first, but slowly warmed up to him and they kissed before he left and promised to rescue her. Leo later rescued her and brought her to the world.
- Festus- Festus is Leo’s pet robotic dragon and closest friend. He was destroyed but later became conscious again and was fixed by Leo. Festus and Leo later rescued Calypso.
- Ella- Ella is an intelligent harpy who is obsessed with the Sibylline Books. Ella was rescued by Percy, Frank, and Hazel from Phineas in Alaska and later brought to Camp Half-Blood. She is the girlfriend of Tyson. She went back to Camp Jupiter to live in New Rome at the end of the series.
- Tyson- Tyson is the cyclops half brother of Percy and son of Poseidon. He helped find Percy and later went back to Camp Half-Blood with Ella. He is close friends with Annabeth and Grover and the girlfriend of Ella. He went back to Camp Jupiter to live in New Rome at the end of the series.
- Iapetus- Bob is a Titan who’s memory was wiped in the River Lethe by Percy, Nico, and Thalia. He later helped Percy and Annabeth find their way through Tartarus and find the Doors of Death. His memory came back and he decided to be good, and was ashamed of his old self. He was later killed by Tartarus. He is close friends with Damasen and has a cat named Small Bob.
- Damasen- Damasen is a giant who was punished and sent to Tartarus by wanting to live peacefully, and was doomed to kill the drakon for eternity. He healed Percy from his curses and made Annabeth a new weapon out of drakon bones. He later sacrificed himself to Tartarus to save Percy and Annabeth. He was close friends with Bob/Iapetus.
- Juniper- Juniper is the girlfriend to grover and she is seen many times throughout the PJO and HoO and ToA series.
- Clovis- Clovis is the head counselor of Hypnos cabin. He is very sleepy and has dream powers.
- Butch Walker- Butch is the head counselor of Iris cabin. He helped rescue Piper, Jason, and Leo and is great with Pegasi.
- Nyssa Barrera- Nyssa is a member of Hephaestus cabin.
- Jules-Albert- Jules Albert is Nico’s French zombie chauffeur, given to him by Hades. He drives Dakota, Leila, and Nico to Camp Half Blood.
- Jake Mason- Jake is the former head counselor of Hephaestus cabin. He let Leo take over due to his injuries.
- Grover Underwood- A satyr who is best friends with Percy. He helped give Reyna the message from Annabeth and tried contacting Percy with his empathy link.
- Malcolm Pace- The head counselor of Athena’s cabin while Annabeth was away.
- Katie Gardner- A daughter of Demeter. She helped calm nature spirits.
- Argus- A 100-eyed employee at Camp Half Blood.
- Chuck Hedge- The satyr son of Coach Hedge and Mellie.
- Clarisse La Rue- Clarisse is the head counselor of Ares cabin. Clarisse took care of Mellie at Camp and delivered her baby.
- Miranda Gardiner- Miranda is the head counselor of Demeter’s cabin.
- Gwendolyn- Gwen is the former Centurion of the fifth cohort. She died and came back to life due to Death being captured. She retired being a Centurion to attend New Rome University.
- Bryce Lawrence- Bryce is a legacy of Orcus who killed his own centurion, but was reinstated as probatio by Octavian. He tried to arrest Nico and Reyna but he was turned into a ghost by Nico.
- Leila- Leila is a daughter of Ceres and centurion of the fourth cohort. She helped Dakota betray Octavian and Michael.
- Bobby- Bobby is a demigod at Camp Jupiter and one of Jason’s friends.
- Marcus- Marcus is a demigod at Camp Jupiter and légionnaire of the fifth cohort. He was an old friend of Jason.
- Jacob- Jacob is a member of the fifth cohort and the Eagle bearer of the twelfth legion.
- Nathan- Nathan is a demigod at Camp Jupiter and one of Jason’s old friends.
- Larry (Legionnaire)- Larry is a son of Mercury, Centurion of the second cohort, and a member of the Senate.
- Hank (Legionnaire)- Hank is a Centurion of the third cohort and a member of the Senate.
- Vitellius- Vitellius is a lar at Camp Jupiter and a legacy of Asclepius. He is very grumpy and lectured Frank.
- Don- Don is a faun at Camp Jupiter who is a reluctant friend of Hazel. Don always asked everyone at Camp for spare change.
- Julia- Julia is a demigod or legacy living in New Rome and is a little girl, an assistant of Terminus. She wants to be Percy Jackson when she grows up.
- Kinzie- Kinzie is an Amazon girl who detained Frank, Percy, and Hazel, and later flirted with Percy. She was killed by Orion.
- Phoebe- Phoebe is a Hunter of Artemis who was killed by Orion.
- Fleecy- A cloud nymph who was an assistant for Iris.
- Michael Varus- A son of Janus and former praetor who lost Camp Jupiter’s eagle in Alaska in the 1980s. Michael later appeared as a ghost in Ithaca and injured Jason.
- Beryl Grace- Beryl is the Mother of Thalia and Jason. She turned into a mania and appeared to Jason in Ithaca.
- Grandma Zhang- Grandma Zhang is Franks grandmother. She is a legacy of Poseidon and told Frank that he was a demigod. Her house was burned down by ogres and her fate is unknown.
- Jane- Jane is the former assistant of Pipers dad.
- Isabel- The leader of the popular clique at the Wilderness school. She and her friends bullied Piper for being Cherokee.
- Dylan- Dylan is a student at the Wilderness school who was actually a storm spirit. He was created when Typhon was killed and tried to kill Jason and his friends multiple times.
- Antinous- Antinous is one of the suitors of Penelope who was killed by Odysseus. He appeared as a ghost to the Heroes at Ithaca.
- Hasdrubal- Hasdrubal is a former king of Carthage. He appeared as a ghost to the Heroes at Ithaca.
- Hippias- A tyrant of Athens who appeared to the heroes as a ghost at Ithaca.
- Eurymachus- Eurymachus is one of Penelope's suitors who was killed by Odysseus. He appeared as a ghost to the demigods in Ithaca, and was killed by Antinous after giving too much information to Jason.
- Zeus- Zeus is the King of the Gods and father of Thalia. Zeus helped the demigods defeat the giants and helped Jason kill Porphyrion. Zeus also punished Apollo by turning him into a mortal. He smacked the Argo II back to Camp Half Blood.
- Hera- Hera is the goddess of marriage and wife of Zeus. She was kidnapped and was almost forced to marry Porphyrion but was saved by Jason, Leo, Piper, and Thalia. Thalia was angry at her for wiping Jason’s memories and being a terrible person.
- Poseidon- Poseidon is the god of the sea and the Earthshaker. He helped Percy kill Otis and Ephialtes. He is Percy’s father.
- Hades- Hades is the god of the Underworld and father of Nico. He appeared to Nico in Évora telling him he wanted him to be happy and also gave him a zombie chauffeur, Jules-Albert. He later helped Hazel kill Alcyoneus.
- Hermes- Hermes is the messenger of the gods. He helped kill Hippolytos.
- Athena- Athena is the goddess of wisdom and the mother of Annabeth. Athena gave Reyna strength to kill Orion and later helped Annabeth kill Enceladus.
- Aphrodite- Aphrodite is the goddess of beauty and the mother of Piper. She blessed Piper into becoming pretty and later told her that Gaea was the true enemy. She said that she loved Tristan, but couldn’t reveal her godhood to him. She later helped Piper kill Periboia and appeared to Piper, Annabeth, and Hazel in Atlanta.
- Hephaestus- Hephaestus is the god of metalworking the father of Leo. He appeared to him in a dream and told him about the giants. He also transported Festus’s head back to bunker 9. He later helped Leo kill several giants.
- Ares- Ares is the god of war and father of Clarisse. Ares’s voice appeared in Frank’s head telling him what to do and later helped him kill giants.
- Demeter- Demeter is the goddess of the Harvest. She helped kill giants.
- Apollo- Apollo is the god of music and poetry. He gave Leo the Curse of Delos and helped him find Asclepius. Apollo helped kill giants and was later punished by Zeus for revealing the prophecy too soon and letting Octavian pursue his path.
- Artemis- Artemis is the goddess of the hunt. She helped kill giants with Apollo.
- Hercules- Hercules is a hero turned minor god who guarded the Mare Nostrum. He ordered the demigods to get the cornucopia from Achelous, but was later buried by the cornucopias food and not given the horn.
- Asclepius- Asclepius is the god of medicine. He made Leo the Physician’s cure and diagnosed Jason with being nearsighted.
- Iris- Iris is the goddess of the rainbow. She owns an organic store for food and gives advice to demigods. She helped Percy send an iris message and gave Frank advice to reject the gods and his destiny, to seek Phineas, and how to defeat the basilisks.
- Hecate- Hecate is the goddess of magic. She takes Hazel to the crossroads and shows her choices, gives her Gale, and teaches her how to manipulate the mist. Hecate later shows up and helps Hazel and the demigods defeat Clytius and save Percy and Annabeth.
- Thanatos- Thanatos is the god of death. Thanatos was captured by Alcyoneus, preventing death from happening and letting the giants close the Doors of Death. He was freed by Frank. Thanatos told the demigods to go to Rome. He also let Hazel and any of the demigods who almost died to stay alive.
- Aeolus- Aeolus is the god of weather. He has gone crazy over the years and his assistants never last long, including Mellie. He was commanded by Gaea to kill Piper, Jason, and Leo.
- Boreas- Boreas is the god of the North Wind. Jason, Piper, and Leo go to him for advice and changes into his Roman form. He releases them, thinking that they are bound to die.
- Khione- Khione is the goddess of snow. Leo has a crush on her, but she attempts to kill Leo, Piper, and Jason numerous times on their quest to free Hera. She was the one who tricked Zeus into closing Olympus. Later, she attacks the Argo II on their way to Epirus, freezes the crew, and blasts Leo to Ogygia. She is defeated by Piper.
- Calais and Zethes- Calais and Zethes are sons of Boreas. Zethes has a crush on Piper and the two help Khione kill the demigods. They are incinerated by Festus’s head.
- Notus- Notus is the god of the South wind. After Jason chooses to be Greek, Auster switches to his Greek form, Notus, and sends the heroes to Malta to rescue Leo.
- Kymopoleia- Kymopoleia is a minor sea god who sided with Gaea. After Jason promised to give her recognition, she helped Jason kill Polybotes and save Percy.
- Eris- Eris is the goddess of mischief. She is shown to Percy and Annabeth by Nyx in Tartarus, and Eris claims that she is the worst terror.
- Geras- Geras is the god of old age. He claims that he is the worst terror.
- Nike- Nike is the goddess of victory. She keeps switching between her Greek form and her Roman form, Victoria, when battled by the demigods. She is later captured and she tells the demigods that one of them will die. She gives advice to Leo.
- Nemesis- Nemesis is the goddess of revenge. She appears to Leo and Hazel in Utah and gives Leo a fortune cookie, which he will use when needed. Nemesis appears as someone you hate.
- Achelous- Achelous is the god of the river Achelous. His horn is stolen by Piper, Jason, and Percy.
- Pluto- Pluto is the god of the Underworld , Roman form of Hades, and the father of Hazel. Pluto appears to Hazel twice, once on her birthday, giving her art supplies, and after she defeats Sciron, complimenting her and letting her stay alive. He also warns her about Pasiphaë.
- Mars- Mars is the god of war, Roman form of Ares, and the father of Frank. Mars appears to Frank at Camp, congratulating him and claiming him, and later appears to him at his grandmothers house, telling him about his gift and that his Grandma was dying. He also gives Frank a book, The Art of War, as a present. His voice later appears in Franks head, telling him what to do, gives him the Blessing of Mars, and a growth spurt.
- Bacchus- Bacchus is the god of wine, Roman form of Dionysus, father of Dakota. Bacchus appears to the Seven in Kansas and later helps them kill Otis and Ephialtes.
- Juno- Juno is the goddess of marriage, wife of Jupiter, and Roman form of Hera. She cursed Frank so his life is tied to a piece of firewood and takes away Percy’s and Jason’s memories. Juno appears in both of their dreams and helps Percy get to Camp Jupiter.
- Minerva- Minerva is the goddess of wisdom, Roman form of Athena. Minerva is very angry and bitter because of how the Romans saw and treated her. Minerva appears to Annabeth in a dream, angry at her, telling her that she is not her daughter. Unlike Athena, Minerva is a virgin goddess.
- Venus- Venus is the goddess of beauty, mother of Michael, and Roman form of Aphrodite. She appears to Hazel, Annabeth, and Piper in Atlanta as both her Greek and Roman form, because love is universal.
- Cupid- Cupid is the god of love and Roman form of Eros. Cupid appears to Jason and Nico in Salona, forces Jason to admit things to himself, and forces Nico to come out in front of Jason. After Nico does, he gives them Diocletian's scepter.
- Favonius- Favonius is the god of the West Wind and the Roman form of Zephyros. He appears to Nico and Jason in Split, and turns them into wind, sending them to Salona.
- Auster- Auster is the god of the South wind, Roman form of Notus. Jason visits Auster for five days straight, but he doesn’t give in to Jason until Jason chooses Greek or Roman. When Jason chooses Greek, he turns into his Greek form.
- Aquilon- Aquilon is the god of the North Wind, Roman form of Boreas. He turns into his Roman form when Jason adresses him as such.
- Eurus- Eurus is the god of the East Wind. Eurus joins the gods in killing the giants in Athens.
- Victoria- Victoria is the goddess of victory, Roman form of Nike. The demigods face her while fighting against her Greek form in Olympia. They defeat her, and Nike/Victoria tell them that one of them will die. The demigods kidnap her and take her to the Argo II. Victoria gives advice to Leo.
- Terminus- Terminus is the god of boundaries, and guards New Rome on the Pomeranian line with a little girl, Julia, preventing weapons from entering the premises. Terminus helps kill Polybotes with Percy and was very suspicious of Greek demigods.
- Rhea Silvia- Rhea Silvia is the wife of Tiberinus, and the mother of Romulus and Remus. Rhea Silvia helps Annabeth on Hercules quest to find the Athena Parthenos and talks to her about the buildings. She looks like Audrey Hepburn.
- Tiberinus- Tiberinus is the god of the Tiber river, the wife of Rhea Silvia, and the person who saved her children, Romulus and Remus. He helped Annabeth find the Athena Parthenos and talked to her about Rome, and drove her around the city. He looks like Gregory Peck.
- Bellona- Bellona is a Roman goddess of war and the mother of Reyna and Hylla. Bellona gave Reyna strength to defeat Orion.
- Gaea- Gaea is the primordial goddess of the Earth. Gaea was asleep for millennia but she was slowly waking up and rising. Gaea began to raise her children, the Giants, and had them captured Hera and Thanatos, and later, Nico. She also had them open the Doors of Death. She also manipulated Hazels mother into trying to raise Alcyoneus and Leo into killing his mother. She was risen by the giants on August 1st, and she rose at Camp Half-Blood, but was defeated by Leo, Jason, Piper, and Octavian.
- Tartarus (primordial)- Tartarus is the primordial of Tartarus, the deepest pit of the Underworld, where the monsters and Titans went, and the father of all giants. Tartarus’s human form has no face, because his face is a vacuum, and it sucks souls to be disintegrated. Tartarus took a human form when Percy and Annabeth were at the Doors of Death and killed three titans and dozens of monsters. Percy and Annabeth escaped him, But he probably killed Bob and Damasen.
- Nyx- Nyx is the primordial of night, and is very terrifying. She lives in Tartarus and has a chariot with two horses: Shade and Shadow. Everyone in Tartarus is afraid of her and she threatened to eat Percy and Annabeth. The two managed to escape her by having her children’s fight over who is the best terror, and then ran through her mansion: the Mansion of Night, where if mortal eyes look, you will be disintegrated.
- Akhlys- Akhlys is the daughter of Nyx and Chaos, and the primordial of misery and poison. She lives in Tartarus. Akhlys is miserable and has scratches on her face. She gave Percy and Annabeth Death Mist, which shielded them From monsters. Akhlys ended up betraying the two and trying to kill them, but Percy saved them by trying to choke her on her own poison. Akhlys then fled in fear.
- Ourae- The Ourae are the primordials of mountains. They are all around twenty feet tall. The Ourae of the Appenine mountains attacked the Argo II as they flew through.
- Porphyrion- Porphyrion is the king of the giants by the bane of Zeus/Jupiter. Porphyrion kidnapped Hera and wanted to marry her, taking her to the Wolf House, so Jason, Leo, and Piper went on a quest to free her, and they killed him with the help of Hera. He was later killed again by Jason and Zeus.
- Enceladus- Enceladus is the bane of Athena. Enceladus kidnapped Pipers father, Tristan, as a trap to distract them from Porphyrion, and kept him at Mount Diablo. He sent Earthborns to kill Jason, Piper, and Leo, Burbank they killed them, and Jason killed him with a lightning strike from Zeus. Annabeth and Athena later killed him again.
- Polybotes- Polybotes is the bane of Poseidon/Neptune. Polybotes rose and planned to attack Camp Jupiter. When he attacked, he and his forces were defeated by the Romans and the Amazon’s, and he was killed by Percy and Terminus. He was killed again by Jason and Kymopoleia.
- Alcyoneus- Alcyoneus is the bane of Hades/Pluto. He almost rose in the 1940s by Gaea and Marie Levesque, but Hazel stopped them. He later rose again in the 1980s and took Camp Jupiter’s eagle, and later captured Thanatos. He was defeated by Percy, Frank, Hazel, and Arion, and later by Hazel, Arion, and Pluto.
- Otis and Ephialtes- Otis and Ephialtes are the banes of Dionysus/Bacchus. They kidnapped Nico and put him in a bronze jar, planning to destroy Rome and Annabeth with the help of Arachne. They were killed by Percy, Jason, Bacchus, and the Argo II, and killed again by Percy and Poseidon.
- Clytius- Clytius is the bane of Hecate/Trivia. Clytius guarded the doors of death and could only be destroyed with fire, since he was anti-magic. He was killed by Leo, Nico, Hazel, Frank, Jason, Piper, and Hecate in Epirus.
- Mimas- Mimas is the bane of Hephaestus/Vulcan. Since he was the bane of Hephaestus, nothing can go right around him and you need to use emotion to kill him, not logic. He was killed by Annabeth and Piper in Sparta.
- Thoon- Thoon is the bane of the fates. He captured Percy and Annabeth as sacrifices for Gaea, but his hand was chopped off by Piper and was later faced off by Percy. Thoon was killed by the fates.
- Periboia- Periboia is the princess of the giants and the bane of Aphrodite/Venus. Periboia is immune to Charmspeak, and is the daughter of Porphyrion. She also wounded Annabeth, having her spill the blood of Olympus. She was killed by Piper and Aphrodite.
- Hippolytos- Hippolytos is the bane of Hermes/Mercury. He was killed at Athens by the seven and Hermes.
- Orion- Orion is the bane of Apollo and Artemis/Diana. Orion was a male who joined the Hunters of Artemis and fell in love with Artemis, but was driven mad by Apollo and later killed by Gaea. Orion got a grudge for all female heroes and went on a quest to kill hem. Orion killed many of the Hunters, like Naomi, Phoebe, Kinzie, and Celyn, and was defeated by Reyna, with strength from Bellona and Athena.
- Medea- Medea is a sorceress granddaughter of Helios who kidnapped Coach Hedge and owned a department store. She Charmspeaked Leo and Jason into turning against each other, but she was defeated by Piper.
- Midas- King Midas was a man with a curse that made whatever he touched turn to gold. He and his son lived in a mansion in Omaha, Nebraska. Midas turned Piper and Jason into gold, but they are restored by Jason and his lightning.
- Lityerses- Lityerses is the son of King Midas and lives with him in Omaha. He was forced to fight Jason, but Jason tricked Midas into turning him to gold.
- Euryale and Stheno- Euryale and Stheno are Medusa’s sisters. They rose again from Erebos after they faded and hunted Percy for months in revenge for killing Medusa.
- Phineas- Phineas is a demigod son of Neptune. Phineas could see the past and future and starved his harpies, since they couldn’t die, and wanted Ella. Percy went to see him because he could cure his amnesia, and they did a challenge, having him drink Gorgons blood: one that will kill him and one that will cure his blindness. Phineas was killed but he cursed Percy.
- Otrera- Otrera is the original queen of the Amazons, put back to life by Gaea to overthrow Hylla, and help destroy Camp Jupiter with Polybotes. Otrera was overthrown by Hylla.
- Aphros and Bythos- Aphros and Bythos are the ichthyocentaur sons of Kronos, and the instructors of the Sea version of Camp Half-Blood. They kept Hazel, Frank, and Leo from drowning and sent his heroes on a quest to save the sea creatures trapped by Phorcys.
- Skolopendra- The Skolopendra is the child of Phorcys and Keto. It tried to attack the Argo II in the ocean and almost destroyed the ship.
- Arachne- Arachne is the descendant of spiders and thought she was better at weaving than Athena. Arachne has a grudge against Athena kids and kept the Athena Parthenos, and tried to kill Annabeth. Annabeth defeated her and got the statue, but Arachne tied a rope on Annabeths ankle, dragging her and Percy into Tartarus. She was later killed by Percy in Tartarus.
- Echo- Echo is a nymph who can only repeat what is said to her Andvari was in love with Narcissus. Echo helps Hazel and Leo and Leo becomes attracted to Echo. When the two leave, Leo asks Echo to come with him, but Echo decided to stay behind to help Narcissus, and kisses Leo on the cheek. Leo forgets her real face when she leaves.
- Narcissus- Narcissus was a man cursed by Nemesis to fall in love with his reflection. Hazel and Leo seemed him in Utah to steal his mirror and use the celestial bronze to repair the ship. Leo tricked the nymphs to fall in love with him instead and leave Narcissus. Leo and Hazel managed to get the mirror and escape him.
- Lycaon- Lycaon is the descendant of werewolves. He was defeated first by Leo, Piper, Jason, and the Hunters, and again by Nico, Reyna, and Coach’s Hedge.
- Chrysaor- Chrysaor is a son of Poseidon and Medusa who has a crew of dolphin hybrids, and tried to loot the Argo II. He was defeated by Percy and Frank and they sank the ship, using it as a tribute to Bacchus.
- Pasiphaë- Pasiphaë is a sorceress, the mother of the Minotaur, and former wife of King Minos. She guarded the Doors of Death at Epirus and later rebuilt the Labyrinth. Pasiphaë was defeated by Hazel when Hazel manipulated the mist to make her drop down a trapdoor.
- Sciron- Sciron is a bandit and pirate who tricked people by having them clean his feet, then kicking them out the window, being eaten by a giant turtle. Sciron encountered the Argo II and demanded the Athena Parthenos. Hazel and Jason cleaned his feet, but Hazel manipulated the mist and turned the boat away from the cliff without Sciron noticing. Sciron was then eaten by his own turtle.
- Arai- The Arai are the daughters of Nyx and the embodiments of curses. They attacked Percy and Annabeth in Tartarus. Every time one of them was killed, Percy and Annabeth got cursed. Eventually, Percy almost died from all of the curses, but Iapetus saved them. Later, Arai chased Percy and Annabeth through the Mansion of Night but they lost them by jumping over the river Acheron.
- Kekrops- Kekrops is the snake hybrid founder of Athens, who was on the side of Gaea. Kekrops led the Seven to the Parthenon and planned to sacrifice them to the giants, but Piper charmspeaked him into not doing it.
- Hagno- Hagno is a nymph who took care of Zeus when he was a baby. Hagno went crazy over the years and allied with Gaea, and tried to drown Percy, Piper, and Jason.
- Maeonian Drakon- The Drakon is the monster that Damasen is condemned to kill for eternity in Tartarus, and almost killed Annabeth and Percy.
- Kerkopes- The Kerkopes are two thieves, children of Oceanus and Theia, and we’re turned into monkeys by Zeus. They stole Leo’s tool belt, but Leo had them go to New York and distract the Romans by stealing their things, which was successful.
- Nikai- Nikai are creatures who serve the goddess Nike/Victoria. They were killed by Leo, Jason, Hazel, and Frank, in order to subdue her.
- The Lost Hero – October 12, 2010
- The Son of Neptune – October 4, 2011
- The Mark of Athena – October 2, 2012
- The House of Hades – October 8, 2013
- The Blood of Olympus – October 7, 2014
- The Demigod Diaries - August 14, 2012
- The Lost Hero was made available as an audiobook read by Joshua Swanson on October 12, 2010, with a play length of 16 hours and 34 minutes.
- The Son of Neptune was made available as an audiobook, read by Joshua Swanson, on October 11, 2011 (a week after the release of the book), with a play length of 13 hours and 27 minutes.
- The Mark of Athena was made available as an audiobook read by Joshua Swanson on October 2, 2012, with a play length of 15 hours and 9 minutes.
- The House of Hades was made available as an audiobook read by Nick Chamian on October 7, 2013 with a play length of 17 hours 32 minutes
- The Blood of Olympus was made available as an audiobook read by Nick Chamian on October 6, 2014 with a play length of 14 hours 26 minutes
- Two Giants have appeared in each book except the Blood of Olympus.
- The Lost Hero is the only book in the series where one of the two giants that appears in the book lives.
- The Son of Neptune is the only book in which Leo and Piper do not narrate a chapter.
- The House of Hades is the only book in which all members of the Seven receives a chapter in their point of view.
- The Blood of Olympus is the only book where several giants appear within the same book. They are Porphyrion, Polybotes, Orion, Mimas, Thoon, Hippolytos, Enceladus, Periboia, Otis and Ephialtes.
- From The Son of Neptune onwards, each character received four chapters in their point of view.
- The Heroes of Olympus series is the only series in the Percy Jackson franchise to not be written in the first person narrative format, instead being written in the third-person format.
- Unlike the Percy Jackson series, there are no chapter titles in The Heroes of Olympus books. Instead, Roman numerals are used, a nod to the addition of its mythology in the series.
- The Lost Hero is the only Heroes of Olympus Book that doesn't have "of" in the title.