Thalia Grace

"I'm honoring a friend. I must join the hunt, Percy. I haven't known peace since...since Half-Blood Hill. I finally feel like I have a home. But you're a hero. You will be the one of the prophecy."

- Thalia to Percy

Thalia Grace (Θάλεια Γκρέις in Ancient Greek) is the Greek demigod daughter of Zeus and the sister of Jason Grace. She was transformed into a pine tree on Half-Blood Hill, which still stands as the official border for Camp Half-Blood to this day. Later she was revived with the Golden Fleece. She is currently the lieutenant of the Hunters of Artemis. Her mother passed away in a car accident.

History
Thalia is the daughter of Zeus and Ms. Grace, a beautiful actress in her prime, but had spiraled into alcoholism in later years. After Thalia's birth, Zeus left the Grace household (as is custom for gods) and Ms. Grace became unstable. When Thalia was about seven, Zeus returned in his Roman aspect, Jupiter. Thalia noted that Zeus in his Roman form was more fatherly and sterner towards her and spoke Latin very often. Within the year, Jason Grace, a Roman demigod, was born. Zeus felt obliged to keep appearing at Ms. Grace's house to please her, but departed a second time because she persisted him incessantly to grant her eternal beauty or a visit to Olympus. When Thalia was nine and Jason was two, their mother took them for a picnic, but when Thalia went to the car for the picnic basket, Hera/Juno took Jason away to Lupa, a she-wolf (and the Roman counterpart of Chiron) at Camp Jupiter. Thalia was heartbroken by her brother's "death" and called his disappearance "the last straw" with her mother and ran away from home, never to return, being this the reason why she only uses her mother's last name (Grace) only when it's extremely necessary.

After running away, Thalia came across another demigod, Luke Castellan, a son of Hermes. They became friends and joined forces to fight the monsters that attacked them. It is assumed by some fans that they started a romantic relationship. When Thalia was twelve, she and 14-year-old Luke found/ran into Annabeth Chase, a plucky blonde-haired seven year old. Luke admired her courage, and gave her his knife, and promised that he wouldn't fail her like her family had. The three of them stayed in different safe houses for protection and shelter, but when their nearest one was destroyed, and Thalia had an injured leg.Luke was forced to return to his home for supplies. The three of them met May Castellan, Luke's mother, and Hermes, Luke's father. As May bandages Thalia's injured leg, Luke and Hermes got into an argument, causing Luke to storm out of the house with Annabeth and Thalia following not too far behind.

Soon they were found by a satyr named Grover Underwood who tried to get them to Camp Half-Blood. Together, the quartet traveled to Camp Half-Blood but were chased by monsters from the Underworld because Hades discovered Thalia's existence and was angered that Zeus had broken his oath not to sire anymore children. They would have escaped, except for the fact that a Cyclops in Brooklyn stopped them so that monsters could catch up. The Cyclops held Thalia, Luke, and Grover in the air to direct the monsters in their direction, but Annabeth saved them by stabbing the Cyclops in the foot.

They also met the Hunters of Artemis, and Zoë Nightshade almost convinced Thalia to join the Hunt, but Thalia refused to leave Luke. She and Zoë argued and Zoë told her that Luke would disappoint her. Once they reached camp, a hoard of monsters attacked (all three Furies and an army of hellhounds), and sacrificing her life on Half-Blood Hill to protect Annabeth, Luke, and Grover. Zeus took pity on his daughter and to prevent her soul from going to Hades, Zeus turned Thalia into a pine tree.

Thalia hated to be called "Thalia Grace" saying, "That's my mother's surname. I don't use it," when it was brought up. Her mother died in a car accident two years before Thalia's resurrection. Thalia might have been named after the ancient Greek muse Thalia of comedy and idyllic poetry or she could have been named after Thalia one of the Three Graces handmaidens of Aphrodite.

The Lightning Thief
Thalia is mentioned once in one of Percy's dreams and during when Grover tells her story. Also, her story is told to Percy Jackson by Annabeth Chase and Grover Underwood

The Sea of Monsters
Thalia's tree is poisoned and the magical borders around camp are failing. Clarisse is sent on a quest to find the Golden Fleece and revive the tree, but Percy and Annabeth don't want to risk Clarisse failing, and also go on a separate quest given by Hermes.

Percy asks Annabeth about Thalia, and whether she would have agreed with Luke. Annabeth was exasperated by the question, and asked Percy if he would ever betray Olympus, which the answer was no. Annabeth said that Thalia got angry with her father a lot, but her personality and heart resembled Percy, saying that they would either be best friends or try to kill each other, and she would never have betrayed her family.

Thalia later appeared in Percy's dreams with stormy green eyes this time, and it is shown she might be the one to bring Kronos back to life.

She comes back to life to conclude the second book after the Golden Fleece purges the poison from her tree and heals the defenses. She appears on the hill the next morning, which is good and bad because it means that Thalia can also be the one of the prophecy and Kronos would take advantage of that. Annabeth, Grover and Chiron are stunned at Thalia's return. Curiously, again, they say she has electric blue eyes, contradicting Percy's first mention of Thalia, when he said she had stormy green eyes.

The Titan's Curse
Grover is sent on a mission to find more half-bloods, and he sends a distress call from a military school, Westover Hall, where he has found two. Thalia, Percy, and Annabeth go to Westover Hall, at the time of the school dance. Thalia manipulated the Mist to trick a resident teacher, but it doesn't convince the vice principal, Dr. Thorn, who is a Manticore. Thorn takes the two demigods Bianca and Nico di Angelo, and Percy goes after them, ignoring Thalia's orders to wait. Thorn is about to take the demigods away when Grover, Thalia and Annabeth arrive, but he is thwarted by Artemis and her hunters. Annabeth tries to protect her friends but ends up disappearing with the Manticore, when he jumps off the cliff. Thalia is upset the hunters have arrived, especially their lieutenant Zoë Nightshade.

Thalia is disgusted when Bianca joins the hunt. Artemis sends the campers and hunters with her brother Apollo to go to Camp Half-Blood. Thalia is frightened when Apollo allows her to drive his sun chariot and terrifies Percy and Grover with her driving. They land in camp where Apollo warns Thalia to be good, knowing something she didn't.

The group is greeted by Chiron and Mr. D, but Percy and Dionysus get into an argument and when Nico distracted Mr. D, Thalia took Percy away and warned him not to get on another god's bad side. Percy is still distressed by Annabeth's disappearance, and Thalia agrees that they should not give up hope on her.

During Capture the Flag, Thalia and Percy are captains. Percy disobeyed Thalia's orders and goes after the flag, but the hunters won after tripping up Percy while he was running back with the flag, resulting with Thalia and Percy getting in a fight. Then the Oracle comes out to the field to find Zoë so she can get a quest, and Chiron has to give it to her. They decide to take two campers and three Hunters. Thalia quickly volunteers. In the end the group is: Zoë, Thalia, Bianca and Grover. Afterwards Percy then follows on Blackjack, a pegasus.

The Hunters, Thalia, and Grover stop at a museum after many hours and from a distance Percy (who followed them) sees the General, Dr Thorn and Luke but gets caught. Percy quickly escapes using Annabeth's cap and notifies Thalia and the others.

Though the group (Zoë and Thalia in particular) was rather surprised and angry (especially Zoe) to realize he had followed them, they are interrupted when the Nemean lion attacks. Nothing is able to pierce its armor, much to the anger of the group. However, Percy discovers a weakness and has an idea to expose it. He finds astronaut food in a gift shop and throws them into the Nemean lion's mouth. It causes the creature to expose his mouth long enough for Zoë and Bianca to finish him off.

Percy soon encounters Apollo again when they get on a train headed west. Apollo advises him to seek out Nereus, the Old Man of the Sea.

Thalia and Percy both sit together, and Percy discovers that Artemis had offered to make her a Hunter, and that is why she didn't like the Hunters. Thalia is quiet at first, and then tells him that she didn't join them because of Luke, and then says that Annabeth had been thinking about joining them, and that he should figure out why. With help from Apollo, they reach a small town in New Mexico. There they are surrounded by the skeletal warriors. Grover then senses Pan's presence and a wild gift from him, the Erymanthian Boar. The boar comes to carry them further on to San Francisco. It takes them to the junkyard of the gods, and Percy meets Ares and talks to Aphrodite (a private conversation which the other questors did not hear).

Eventually, the group enters the junkyard, where Bianca tries to take something for Nico Di Angelo, her brother. However, she awakens a defective prototype of Talos and enters a maintenance hole in his left foot to destroy him. She succeeds in destroying Talos, but goes missing (it is later confirmed that she had died). Zoe and the other members of the group are devastated and search for Bianca to no success. All of them realize that Bianca was the one they would lose in the land without rain, a line mentioned in the prophecy.

They go to see Annabeth's father in San Francisco to ask for help and he lends them his car, which cannot be returned to him as it explodes due to a lightning bolt. Thalia is shaken by this, thinking that Zeus tried to kill her, remembering a part of the prophecy one shall perish by a parent's hand, but Percy comforts her. His words convince her, though not fully, and she manages to stay focused.

They all then climb to the Garden of the Hesperides, where Zoë meets her sisters and is poisoned by the dragon, Ladon. They continue to the place where Atlas used to hold up the sky, and they see Artemis taking on his burden. Annabeth is bound and gagged by Luke and Atlas.

They discover that Kronos used Thalia to twist destiny into her hands, since she would be turning sixteen on the day following the Winter Solstice.

Thalia demands answers from Luke, as to why he has betrayed them all, and Luke tries to pacify her, but she takes out her spear and says," you're not Luke, I don't know who you are anymore." and attacks him.

Thalia and Luke battle each other fiercely, fierce anger on Thalia's side, desperation on Luke's. Thalia gained the upper hand in the battle. Thalia manages to disarm him, saying that he could never beat her. Luke tries to grab her weapon, and as a pure instinctive reflex, Thalia kicks him, and Luke falls off a cliff. She is very shocked and sad, having tears in her eyes having done so, that she just killed Luke, but it is confirmed by Poseidon that the traitorous demigod had survived. Thalia decides to become a hunter of Artemis at the end of the book so she would not be the one of the prophecy.

The Battle of the Labyrinth
Thalia was absent from Camp Half-Blood this summer due to her adventures with Artemis and the Hunters, but she is mentioned once by Percy when talking to Hera, the goddess of marriage, about Zeus and Ms. Grace's past relationship.

The Sword of Hades
Thalia is hunting a golden deer, an animal sacred to Artemis in the city when she bumps into Percy and thinks it is very strange that they bumped into each other and couldn't just be coincidence. Nico soon arrives in a shadow and Thalia knows something is wrong, but cannot react quickly enough as the three demigods are dragged into the Underworld. When they arrive, Persephone makes it known that she was the one that brought them together and needs them for a quest. She claims that Hades has made a new symbol of power in the form of a sword, but it had been stolen. Thalia doesn't want any part of the quest, feeling that a new symbol of power will tilt the balance of power between the Big Three gods. However, if the sword escapes the Underworld, it could fall into the hands of the Titans and be used against them. Thalia agrees to help and the three chase after the sword. They question Sisyphus, but while they ask him about the thief, Thalia holds the boulder and begins pushing it up the hill. She almost makes it before it begins to roll back down the hill. She lets go and the rock rolls back onto Sisyphus who is forced to continue his punishment. Thalia tells them that she almost had it and wanted to try again as if it was part of the punishment. They continue to chase Ethan (who they discover was the thief) and eventually get the sword back. After Nico decides to stay in the Underworld, Thalia invites Percy to a place that has amazing cheeseburgers, which he agrees to.

The Last Olympian
Thalia and the Hunters come to New York City to help the campers fight off Kronos' army, and the Campers are very relieved to see them. Thalia says she has been on a great many adventures, and she, Annabeth, and Percy promise to buy cheeseburgers after the battle and talk about what they've been doing since they last saw each other.

Thalia goes with Percy, Annabeth, and Grover to Mount Olympus to defeat Kronos, but she ends up getting left behind when a statue of Hera that was actually intended to fall upon Annabeth, falls on her instead. However, she appears to have survived and continued to work in Artemis' Hunters, and will presumably be immortal for the rest of time, unless, she falls in love or dies in combat, or if she decides to leave, as her joining the Hunters was only to escape the Great Prophecy.

The Lost Hero
Thalia was mentioned in the beginning to be searching for Percy with the Hunters. Later she was talked about when Jason being a son of Zeus (Roman counterpart: Jupiter) sees a picture of her in Cabin One to which he stares at before Annabeth comes in. After a conversation about Thalia's past life with Annabeth, Jason asks what her last name was to which Annabeth responds "Grace". After remembering his dream about Lupa saying he was their "saving Grace", Jason claims to be her brother. Later on in the story, after Jason meets Lycaon, king of the wolves, Thalia and the Hunters kill some of Lycaon's wolves and managed to injure Lycaon himself before they fled. After sending Piper and Coach Hedge off to Phoebe to heal Piper's injuries Thalia and Jason talk about Jason's life before he disappeared.

After their talk about Jason's life they move on to Jason's quest and how they can get to Aeolus. Thalia shows them the way to Aeolus' palace but splits up with Jason, Piper, and Leo to join the Hunters to protect Hera so that while Jason's group could rescue Piper's dad, the Hunters could protect Hera until Jason arrives. While Jason was away to see Aeolus, Thalia and the Hunters were attacked by Lycaon and his wolves. After Jason gets back with Leo and Piper from saving Piper's dad they meet up with Thalia, who says nothing worked on the cage and they should just leave Hera. Hera and Thalia fight, ending with Thalia leaving, but first she tells Hera every bad thing she did to children of Zeus, ending with Thalia reminding Hera that they are at the spot when Hera took Jason. Later, Jason, Piper, Leo, and Thalia were ambushed by Khione and Thalia was frozen by her, along with the rest of the Hunters. Soon after Thalia was frozen, Leo fought Khione while Jason and Piper battled and defeated the venti and the Earthborns. Quickly after, the Giant Porphyrion was reawakened by the powers of Gaea and Hera was being buried by Gaea. Like the prophecy Jason had received "The forge and dove shall break the cage", Leo, a son of Hephaestus (forge) and Piper, a daughter of Aphrodite (dove), worked together to break Hera's cage with Leo trying to saw off the lock of the cage while Piper used charmspeaking to slow Gaea down.

Meanwhile Jason was distracting the Giant Porphyrion taking him on barehanded. While Jason distracted the giant, Leo and Piper finally broke Hera's cage free. The fight between Jason was quickly over with Jason on the floor and Porphyrion's spear at his chest. Just before Porphyrion struck, Hera quickly assumed her Divine Form while telling everyone to look away. Jason was too tired to comprehend her words quickly enough and was nearly killed. Thalia is then seen telling Hera to save Jason, her anger causing her to spit out words without thinking. After Piper telling him to awaken, he did as everyone around him gathered. As Jason got up, Hera was arguing with Thalia but Piper told them to stop. Before Hera sent Jason, Piper, and Leo back, Thalia had told Jason she would attend the council meeting at camp. At the council, Thalia didn't show up but Chiron had said she had other matters to attend to. Thalia also gave Piper a Hunter of Artemis card to join, but Piper burnt it for Jason.

The Son of Neptune
Although Thalia isn't really mentioned much in The Son of Neptune, there are two parts in which she is either mentioned or appeared. First, in Percy's dream which was a flashback of him in The Titan's Curse, Thalia, Percy and Zoë Nightshade battling on Mt. Tam going against Atlas, Luke, and trying to save Annabeth and Artemis. Percy later sees Zoë dying and rise to the stars. Second, though Thalia was never mentioned, when Hylla, the Queen of Amazons, talks to Hazel she mentions a group of archers that travel around who follow the goddess Artemis, which refers to the Hunters.

Personality
Thalia is a brave, strong and caring girl, though at times she can be stubborn, prideful and assertive much like her father Zeus. She likes to be in charge and give out orders to people and becomes very angry if they are disobeyed (normally by Percy). She also has a bit of a temper, as shown during the Capture the Flag match against the Hunters when Percy left his post causing them to lose the game, so she shocked him with electricity. In some ways she's very similar to Artemis, as she enjoys being around her fellow Hunters and is a great fighter. She is also very loyal to those she trusts and respects, as she is willing to risk her life to protect them (for example, when she sacrificed herself on top of Half-Blood Hill). Unlike her fellow Hunters, Thalia apparently does not mind being around boys, as shown in The Lost Hero.

Chiron once said that Percy and Thalia are very much alike. The only difference is that Thalia is more sure of herself, while Percy has less self-confidence.

Fatal Flaw
Thalia's fatal flaw is that she has a weak resistance when she is offered power (mirroring Zeus's power struggle with his brothers, as she also conflicted with Percy over capture the flag, and when offered by Luke to kill the Ophiotaurus). She is also afraid of heights (acrophobia), a quality Percy finds strange and funny, as her father is the Lord of the Sky. In the short story, The Sword of Hades, in The Demigod Files, she also shows to be very determined. Thalia has trouble abandoning a difficult task that she has started but is not able to complete.

She also feels massive amounts of guilt about her brother going missing when he was little, as she more or less raised him on her own (her mother was an alcoholic, and therefore, unstable); she had believed for years that her mother had either killed or abandoned him on a family vacation. She never told anyone about this when she ran away, not even Luke or Annabeth.

Appearance
"The girl in the lead had spiky black hair and a black leather jacket. She wore a silver circlet on her head like a princess's tiara, which didn't match her skull earrings or her Death over Barbie T-shirt showing a little Barbie doll with an arrow through its head."

- Percy describing Thalia

Thalia has shoulder-length or as sometimes described short, spiky black hair, electric blue eyes (though they are "stormy green" in Percy's dream of her in The Lightning Thief and stormy blue in Percy's dream in The Sea of Monsters), and freckles across her nose. She is described as being "extremely pretty" by fellow campers and Apollo, (in The Titan's Curse, when he says "Man, I hate it when pretty girls get turned into trees, a reference to Apollo's love, Daphne") and is "beautiful", according to Artemis. She wears punk styled clothes. She carries a magic silver bracelet that turns into Aegis, a modified version of Zeus' shield, a gift from Athena. In the books, Percy describes Thalia's shield to have the head of Medusa (bronze) imprinted in the center to strike fear into her opponent's hearts. Many campers and monsters cower at the sight of Medusa's head on Thalia's shield. Thalia also carries a collapsible Mace canister which changes into a spear.

Thalia has been described as "punk" or "goth" with a forever present silver circlet on her head after she became a Hunter of Artemis, which looks like a tiara and according to Percy, symbolizes her position as the first lieutenant of the Hunters of Artemis. As a Hunter of Artemis she wears a silvery parka and camouflage pants. In The Last Olympian she was wearing a "Death to Barbie" T-shirt, with a picture of a barbie doll with an arrow going through its head, following with some skull earrings to go along with her shirt. According to Percy, it did not match the silver circlet in her hair at all.

Thalia makes another appearance in The Lost Hero, where Leo Valdez, upon immediately seeing her, instantly developed a crush on Thalia. He thought Thalia was way too pretty to be Jason's sister, and over the fire, Leo thought she looked like the goddess Khione, just as cold and beautiful. Leo also states that "Nah, (while assuring Jason) we were absolutely destined to meet your hot sister," although Thalia ignored this completely. Of course, Thalia is a Hunter of Artemis and has sworn off boys.

Abilities

 * ADHD: Like all demigods, Thalia possesses inborn supernatural battle reflexes and senses that she uses to analyze the fighting style of her opponent.
 * Fighting Skills: Thalia has shown to be a formidable opponent with her bow and arrows, spear, shield, and her hunting knives. This is shown when she was fighting Iapetus in The Sword of Hades.
 * Dyslexia: since her brain is hard-wired for reading Ancient Greek instead of modern languages.

Demigod Powers
Being the daughter of Zeus; Thalia holds divine authority over the creatures of the sky as well as weather, having powers that are greater than the children of nearly every other god.
 * Aerokinesis: It is assumed that Thalia has aerokinesis (the ability to control air) like her brother Jason Grace, as her father Zeus is Lord of the Sky, but has not used it nor even tried it due to her acrophobia, fear of heights.
 * Atmokinesis: Thalia has a minor degree of control over the weather. It is not even close to matching her father but she may be able to call lightning from the sky as Jason did a couple of times.
 * Electrokinesis: Thalia has electrokinesis (control over electricity and lightning) due to her father, Zeus, the god of lightning. Calling down lightning bolts are a lot harder than emitting static, as it makes Jason ready to faint.
 * Thalia can send powerful amounts of static shock throughout another person's body on contact. She can emit static sparks when she is angry, like Jason, which are described as being blue.


 * Mist Control: Thalia can manipulate the Mist, which means she can make things appear as she wants them to appear to the eyes of a mortal (taught to her by Chiron).
 * Zeus' Authority: Things that belong to her father's domain can do as she asks, such as statues of the angels in The Titan's Curse, which were gifts to her father. They, on her request, flew her and her friends to San Francisco.
 * Immortality: Thalia became Artemis' lieutenant (replacing Zoë Nightshade) in the end of The Titan's Curse, and as a result she is immortal, unless she falls in battle, chooses to leave, or falls in love.

Weapons

 * A copy of Aegis that expands from a shiny silver metal bracelet that she wears on her wrist. The shield is decorated with an image of Medusa's head, which many of the campers and monsters cower from.
 * A collapsible spear that expands from a Mace canister, which she uses to direct her powers.
 * A Hunter's bow, given to her after joining the Hunters.
 * Two hunting knives that she can use to direct electric blasts.

Luke Castellan
It was hinted that before Thalia became a Hunter, she and Luke had a romantic past as she told Zoe that she wouldn't leave Luke behind. That was changed when Luke betrayed Camp Half-Blood in The Lightning Thief. It further either dissolved after Luke poisoned her tree in The Sea of Monsters. In The Titan's Curse, Thalia was noted by Percy that she was crying when she fought with Luke, and she stated that she hadn't known peace since she'd traveled with Annabeth and Luke. One of the reasons Luke betrayed Camp Half-Blood is because he thought it was what Thalia would have wanted, but this is never verified. Also, Luke apparently never got over the fact that Thalia got turned into a tree and her father, Zeus didn't "save" her.

Annabeth Chase
In The Lost Hero, Thalia called Luke and Annabeth her best friends, and it was shown in The Titan's Curse that Thalia was extremely worried about Annabeth when she was abducted and wanted to rescue her. She even took a falling statue for her in The Last Olympian.

Grover
Thalia gets along with Grover reasonably well. Grover was the first Keeper that was supposed to deliver Thalia and the other two half-bloods (Annabeth and Luke) to Camp Half-Blood but failed to do so. As a result, Thalia had been forced to confront a massive amount of monsters that were sent by Hades to hunt them down. Though extremely powerful, Thalia was overwhelmed by their numbers and very nearly perished. However, her father Zeus took pity on her and turned her into a pine tree. The tree, later known as "Thalia's tree," served as a magical barrier to Camp Half-Blood. Once Thalia was alive again, Grover and Thalia's friendship remained strong. She sometimes calls him "Goat Boy."

Percy Jackson
Like their fathers, Thalia and Percy share many similarities. Despite their tendency to fight, they consider each other close friends. It was Percy who was the first male that Thalia hugged after officially becoming a Hunter, saying "I am honoring a friend."

Jason Grace
Jason is Thalia's little brother. It was seen in The Lost Hero that she loved Jason very much and that it almost killed her when Jason was sent away. She also said that what happened with Jason had hurt her so much that she never mentioned Jason to Annabeth or Luke, her two most trusted friends. It was the disappearance of Jason that caused Thalia to finally run away from her broken home.

Ms. Grace
Thalia has a very rough relationship with her mother, and that she was always drunk and acted like a spoiled child. Thalia's lack of patience was part of the reason Thalia ran away; the other reason being losing Jason. Thalia's mother eventually died in a drunk driving accident.

Artemis
Thalia remains loyal and obedient to her sister Artemis. It was stated by Jason that Thalia had found a new family in the Hunters and a new mother in Artemis.

Zeus
Thalia and Zeus aren't close, but in the ending of The Titan's Curse, it was shown that Zeus was proud of Thalia. In The Titan's Curse, Kronos tries to anger Thalia by hitting their car with a bolt of lightning, which would cause Thalia to think her father was after her.

Hera
"That's it, Thalia Grace. I will turn you into an aardvark, so help me—"

- Hera arguing with Thalia in The Lost Hero

Like every illegitimate child of Zeus (except Jason), Thalia is detested by Hera. When they encounter each other, Thalia quarrels with her stepmother. During The Last Olympian, Hera purposely made a statue fall, aiming for Annabeth but Thalia jumped in the way and was pinned down to the ground by the legs. Thus, she was unable to help in the fight against Kronos. In The Lost Hero, Thalia was arguing with Hera about reviving Jason after Jason accidentally saw her divine form.

Apollo
Before Thalia was a hunter, she had thought Apollo was hot. Now that she is a hunter, she probably does not think that anymore.

The Lightning Thief
Although she is never actually mentioned in the movie, her tree appears and she is also mentioned in a deleted scene by Grover, who was played by Brandon T. Jackson.

Etymology
Thalia is a Greek name meaning To flourish, Bloom, which could be tied with the fact that Thalia was a tree and then bloomed out when the Golden Fleece healed her. Thalia is also the name of one of Muses, Thalia, Muse of Comedy and also a Daughter of Zeus. Her last name is tied to her half-siblings, the Graces.

Trivia

 * In the Lost Hero, Thalia was described as "she was maybe fifteen, with black spiky hair, a black leather jacket, and silver jewelry, so she looked kind of goth/punk; but she was caught mid-laugh, and it was clear she was with her two best friends." This must be a mistake, because she was twelve when she was on the run with Luke and Annabeth.
 * She, Bianca di Angelo, and Nico di Angelo are the only Greek Half-Bloods of the Big Three who's aging has been tampered by their godly parent, which differs them from Percy greatly.
 * If Thalia had never been turned into a pine tree, according to her date of birth, she would be 21, so the prophecy would have landed on her, not Percy, but her aging slowed during her time as a tree, so at the conclusion of "The Titan's Curse" she had not yet reached 16 years of age. Consequently, upon joining the Hunters of Artemis, she is a few hours younger than 16 years of age, leaving Percy responsible for the Great Prophecy.
 * Thalia, ironically, has a fear of heights, even though her father, Zeus, is the god of the sky.
 * Thalia's last name wasn't learned until The Last Olympian. According to Percy, it made her seem almost normal, and less mysterious and powerful.
 * In The Lightning Thief, Thalia is said to have stormy-green eyes and when Percy has a dream about her in The Sea of Monsters, she is said to have stormy-blue, but when she is revived from being in tree form and in the following books she has electric-blue eyes.
 * When Thalia meets Apollo for the first time, she refers to him as "hot", and not just because he is the sun god.
 * Thalia loves cheese hamburgers. Everytime she ends a journey with Annabeth or Percy (or both), she'll ask them for hamburgers.
 * The name 'Thalia' is also the name of one of the three Graces, hence her name: 'Thalia Grace'.
 * Thalia is also the name of one of the nine Muses, the Muse of idyllic poetry and comedy.
 * Thalia likes the band Green Day, as seen in The Titan's Curse. It is also noted she hates Jesse Mcartney's music.
 * Leo from The Lost Hero has stated that Thalia is very pretty and the type of girl who wouldn't give Leo the time of day.
 * Thalia is the only known demigod, besides Percy, who could fight Luke on equal terms. She even stated that she was a better fighter than Luke.
 * So far she and her brother, Jason, are the only siblings that are Greek and Roman, as her father visited her mother in both aspects.
 * Thalia is the only living demigod child of the Big Three to not have an official portrait. (She is said by Rick Riordan via Twiter to have a portrait in the works that will be released in "The Demigod Diaries.")