Thalia Grace

Thalia Grace is a fictional supporting character in Rick Riordan's "Percy Jackson and the Olympians" series, as well as a minor character in the Heroes of Olympus, as Jason's sister. She is the demigod daughter of Zeus and Beryl Grace, a famous mortal actress. Thalia has angel-blue eyes ( thought done say she has stormy green), black short spiky hair, and a spray of freckles and a brilliant singing voice.
Background.
Thalia Grace is the daughter of Zeus, one of the "Big Three" Greek Gods, the other two being Poseidon and Hades. Grover Underwood was meant to watch over her when she ran away from home at ten years old, reaching Camp Half-Blood two years later. On her journey, she met Annabeth Chase and Luke Castellan. They had brief encounters with the Hunters of Artemis, and were almost convinced to join, but got angry with Zoe Nightshade and did not join. When she was twelve, they were all attacked by a herd of monsters and caught by a Cyclops in New York. She was captured, but thanks to Annabeth, freed. They ran, but the monsters they had briefly escaped from caught up to them. As they reached Half-Blood Hill, Thalia sent Luke, Grover, and Annabeth over the boundary line while she faced the monsters. She sacrificed her life for her friends, but her father Zeus took pity on her and turned her into a pine tree to preserve her, keeping the rest of the camp safe.
Thalia is a very skilled fighter and as she stated in "The Titan's Curse"; she can always beat Luke in a battle (which Percy has only done once). It is also mentioned throughout the series that she has a punk sort of style, hates Barbie dolls, and likes Green Day. She is always listening to rock bands and is into the punk subculture. Annabeth compares her to Percy, telling him, "You guys are so alike it's scary. Either you would have been best friends, or you would have strangled each other."
Thalia wants to be strong as many people believe, but sometimes is very sensitive. She loves Chiron as a father and believes that her father does not care about her. She deeply respects Poseidon but does not know why. Her favorite god is Pan. She is also known to like cheeseburgers, mentioned twice in the series. She also appears reluctant to use her mother's surname.
Percy Jackson and the Olympians.
"The Lightning Thief".
It is mentioned that Thalia was turned into a pine tree when she risked her life saving her friends atop of Half-Blood Hill, which guarded the boundary line for Camp Half-Blood. She also appeared in Percy's dream. This is one of the first movies that came from the series, in fact it is the first movie. However, she was mentioned a few times, no one in the beginning really knew why. 
"The Sea of Monsters".
Thalia appears in Percy's dreams throughout the book. At the end of "The Sea of Monsters", she is brought back to life by the Golden Fleece which Clarisse places on her pine tree. Zeus gives her a spear which shoots electricity and turns into a small mace canister that fits in her pocket, and Athena gives her a bracelet that turns into Aegis, a shield with the Medusa's head carved into it. It is a replica of her father’s. Thalia is now potentially the child of the prophecy, and Kronos soon takes advantage of the fact. in the sea of monsters Percy uses the sea as a compass to navigate to find the golden sheet which will bring back Thalia as her tree was dying in front of the camp
"The Titan's Curse".
Thalia and Percy quarrel occasionally at camp because their similar personalities tend to clash. Thalia is called "Pinecone Face" by Percy; in retaliation, she calls him "Seaweed Brain", which Percy does not like because it's Annabeth's pet name for him. Thalia goes on the quest to find Artemis, hunt down the dangerous Ophiotaurus Artemis was seeking, and rescue Annabeth, along with Percy, Zoë Nightshade, Bianca di Angelo, and Grover. Thalia, Percy, Zoe, Bianca, and Grover come upon the Junkyard of the Gods and told not to take anything from the junkyard, and just as the group is about to leave, a metal giant rises because Bianca took a figurine of her father, Hades, for Nico. Bianca goes inside of the metal giant and tries to control it, causing the giant to fall over and Bianca to die. Together, they face the Titan General, Atlas's army, and make it to Mount Tamalpais. Already weakened by dragon poison, while facing her father, Atlas, Zoe dies from a final blow from Atlas. There, Thalia fights her old friend Luke, and it seems that she has strong feelings for him, and 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. She escaped this deadly path and turned to her half-sister, Artemis, who made her a Hunter, which would stop her from being sixteen, because whoever becomes a Hunter becomes immortal, unless they shall die in battle, or break the oath by falling in love.
Thalia's tree still remains standing on Half-Blood Hill with the Golden Fleece for energy, and is currently being guarded by a baby dragon called Peleus.
"The Battle of the Labyrinth".
Thalia is 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 Last Olympian".
Thalia and the Hunters of Artemis show up when Percy is looking for someone to defend the Lincoln Tunnel against the Titans. Thalia helps Percy and the other demigods defend Manhattan. At the end of the story, Thalia is still alive, and is granted help in getting more Hunters, because a lot of the Hunters died.
The Heroes of Olympus.
"The Lost Hero".
Thalia shows up at the cave by Aeolus' palace, along with the other hunters. She also fights at the wolf house when the hunters help Leo, Jason, and Piper fight the monsters and the giant.
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 the Amazons, talks to Hazel Levesque she mentions a group of archers that travel around who follow the goddess Artemis, which refers to the Hunters.
The Blood of Olympus.
Thalia and the hunters abducted Reyna Avila Ramírez-Arellano in San Juan and took her to her sister Hylla Ramírez-Arellano. She explained that the hunters and Amazons have been fighting monsters while the demigods have been fighting each other. Orion then attacked their stronghold, killing many Hunters and Amazons though Thalia and Hylla manage to survive. When Nico and Hedge appear, she protects them while they and Reyna shadow travel away. She survives the battle.
The Trials of Apollo.
The Dark Prophecy.
Thalia and the hunters were close by when Apollo signaled for back up. They arrived with Leo and Festus to aid in the rescue operation and she decided to rode on Livia with Apollo, so she could help them to escape. After they fled, the lieutenant of Artemis offered the fallen god Moonwater to regain his strength. Then she finally says Artemis was forbidden by Zeus from interfering with Apollo but still ordered the Hunters to help him. They three then head back to the Waystation.
She and Apollo ride Livia in to the Waystation and she goes to stop her hunters from possibly killing Lityerses. After they rally everyone for battle she requests more arrows from Emmie.
Halfway through the battle, she along with Lit, Calypso and Josephine stood by as Commodus had Emmie, Leo and Georgie hostage. Apollo asks her to stand down before he goes not his Divine Form and blinds the emperor. She takes out one of his guards and searched the surrounding area for Commodus with no results. After that, she sits with everybody and discuss about the prophecy revealed by Meg, saying that the Hunters can't help because they just have one mission to finish.
Before she and the Hunters leave, she asks Apollo to say hello to Jason and Piper. In return, the god asks her to say hi to Artemis for him, which she agrees to do.
Fatal flaw.
It is implied that she has a weak resistance when she is offered power. She is also afraid of heights, a quality Percy finds strange, as her father is the Lord of the Sky. She is also extremely determined, as revealed in the short story "The Sword of Hades" in "The Demigod Files". She has trouble abandoning a difficult task that she has started but is not able to complete.