Percy is usually portrayed as being good-natured, friendly, and often willing to risk his life to save his friends, strangers, and even his enemies. Percy also has a very sarcastic sense of humor and is wry in his writing style. Percy seems to dislike titles being given to him as he always tells the horses not to call him "boss" whenever they talk to him. Percy is occasionally shown to be fairly childish, such as when he compares Artemis' chariot to Santa's sleigh (though this is possibly due more to humor and cultural reference than to illustrate immaturity). He also has a sense of loyalty for he turned down godhood when it was offered to him in order to remain with his friends, Annabeth and Grover.
Percy Jackson was raised by his mother as a single child. When he was at the right age for his powers to be discovered, he was found by a satyr named Grover, who disguised himself as a human and befriended Percy at Percy's boarding school, Yancy Academy. He was Percy's best friend at this school. On a school field trip, one of Percy's school teachers, Mrs. Dodds, turns out to be a Fury and attacks him. Mr. Brunner, Percy's favorite teacher, who turns out to be a Centaur named Chiron, throws him a gold pen which turns into a sword (later on mentioned as Anaklusmos (Riptide). Percy defeats Mrs. Dodds, but afterwards, no mortals remember that she ever existed, and Grover and Mr. Brunner pretend not to remember her.
After some events,Percy ends up at Camp Half-Blood. He is at first sent to Hermes' Cabin, led by a camper Luke Castellan (who becomes evil at the end of the book.) He tries to kill Percy with a poisonous scorpion. Percy barely lives. After a capture the flag game, he is found out to be the son of Poseidon, which means Poseidon broke an oath in which the big three (Zeus, Poseidon and Hades) swore not to have any children with mortal women.
Annabeth Chase is the half-blood daughter of the Goddess Athena and the mortal man, Frederick Chase. Since she is a daughter of Athena, she was not physically born, but is a literal 'brain child', and was delivered to her father in a golden basket by Zephyr, the West Wind. When she was seven she ran away from home and on her journey she met Luke, Thalia, and Grover Underwood. Together, they travelled to Camp Half-Blood, running from and fighting hordes of monsters. Upon arrival at the borders of Camp Half-Blood, Thalia sacrificed her life so that Luke and Annabeth could safely get into camp when attacked by a monster. She lived at Camp Half-Blood for five years (not including the years after Percy Jackson arrived).
It has been shown that Annabeth does not get along with her father and step-family. According to her, her father was not very happy when she first appeared, and tends to look at her as a nuisance. Her step-siblings appear to dislike her and feel afraid of her. Her stepmother seems to hate her, and feels that she endangers the family by being only half-mortal.
She has a magical Yankee's cap given by her mother Athena, which makes her turn invisible while she wears it, and a celestial bronze knife, which was a gift from Luke.
Rachel originally appeared at Hoover Dam in The Titan's Curse. She saves Percy Jackson from a squad of skeletons who are about to attack him by sending him into a restroom behind her and sending the skeletons to the generators.
In The Battle of the Labyrinth we get to learn more of her background. Rachel grows more and more fond of Percy as a good friend throughout The Battle of the Labyrinth, just as Annabeth grows very jealous of her. Rachel sees that Percy and Annabeth seem to have a lot more feelings for each other then they will admit. She can also see that Annabeth is jealous of her hanging out with Percy and tries to give him the hint that Annabeth likes him, too, but Percy doesn't seem to get it. It is, however, made clear that Percy does have some feelings towards Annabeth and that there might be a spark for Rachel forming.
Between the fourth and fifth book Percy and Rachel become better friends.
He is the son of Hades and brother to Bianca. His sister died in The Titan's Curse from a defective Talos. He can tell if someone he loves is in the Underworld and if they are being judged. He also know when a mortal or half-blood is dead, from a buzzing in his ears. He can also call spirits from the dead and/or communicate with them. He was angry with Percy Jackson for letting his sister die and tries to get back at him, but at the end of The Battle of the Labyrinth he seems to come up with his own mutual agreement. Nico has discovered a way to defeat the titans, but the book ends before this is told. In The Titan's Curse, it is said that Nico and Bianca are more than 70 years old due to the Lotus Casino, which means Hades was the only god among the Big Three that kept his promise not to have children after World War II.