Percy is good-natured, friendly, brave, and willing to risk his life to save his friends, strangers, and even his enemies. He is angered by Thalia Grace because their personalities are similar, they argue even though they are good friends. Percy has a sarcastic sense of humor and is funny in his writing.
Percy dislikes nicknames or titles being given to him, with the exception of those given by Annabeth Chase. He always tells horses, which his father is said to have created, not to call him "boss" or "lord" whenever they talk to him but does not get annoyed when Annabeth calls him "seaweed-brain". His fatal flaw is loyalty. Percy, like most boys his age, has trouble talking to girls. Athena said that Percy's fatal flaw is personal loyalty. He will do anything to save his friends and loved ones. Kronos uses this flaw to control him several times in the series, such as when his mother is held hostage when Grover is trapped on Polyphemus' island, and when Annabeth is kidnapped.
Perseus "Percy" Jackson is introduced as a troubled 12-year-old boy who is a student with dyslexia, which makes it hard for him to read, and ADHD (Attention Deficit Hyperactivity Disorder), which makes it hard for him to pay attention. He has been kicked out of every single school he has gone to. He has just been kicked out of Yancy Academy in Upstate New York. He has few friends at the school, his best friend being Grover Underwood, a satyr (half human, half goat) disguised as a human.
During a field trip to a museum, Percy is annoyed by the school bully, Nancy Bobofit, for intimidating Grover and in a fit of rage, accidentally summons water from a nearby fountain to pull her in, but he didn't know how he did it. Mrs. Dodds, his pre-algebra teacher, witnesses the whole scene and tells him to follow her into the museum for a "talk." Suddenly, she turns into the Fury Alecto, and attacks him, looking like a bat-like figure with humongous fangs. Percy manages to destroy her using Mr. Brunner’s pen, Riptide, that transforms into a sword in his hand. However, when Percy returns to the bus and asks his classmates, nobody remembers Mrs. Dodds because of a force known as the Mist. The only person that may remember her was Grover, who always hesitated before he answered. Everybody else believed her name was Mrs. Kerr.
Percy asks what just happened, and Mr. Brunner says he saw nothing and that Percy should bring his own things to write with in the future. Percy asks his classmates about it, but they act like there was never any "Mrs. Dodds". Percy sees that Grover is lying when he hesitates whenever Percy mentions "Mrs. Dodds". Percy overhears Grover and Mr. Brunner talking about him later in the school year. As he is going home on a bus with Grover, he sees three old ladies knitting a giant sock. Grover looks really worried about them. Percy leaves Grover at the bus stop and takes a taxi to his apartment on his own.
His mother arrives soon after he gets home and tells him that they can go to Montauk Beach for the entire weekend. Percy is happy that something good has finally happened and packs his things and leaves. At the beach, Percy wakes up in the middle of the night in a storm and is shocked to see Grover coming to him and his mom, telling them to leave. Percy is confused, but his mother takes them both in the car and drives quickly away.
After hearing a growl behind them, his mom arrives at a hill and pushes both boys out of the car. Percy sees that the thing chasing them is a Minotaur, who charges them. Percy and his mom dodge it, but Grover is too slow and the Minotaur charges him. Percy manages to defeat the Minotaur, but not before it dissolves Percy's mother into light and sends her to the Underworld. Percy drags Grover up the hill to Camp Half-Blood, where Percy passes out on the doorway of the Big House.
Percy learns that Camp Half-Blood is a place for demigods like him. He meets a future friend, Annabeth Chase. He learns that Mr. Brunner is actually the centaur (half human, half horse) Chiron, the camp's activity director. He also learns that the director of the camp is the god Dionysus. He is introduced to Luke Castellan, the leader of the Hermes cabin. Later, in a capture-the-flag game, Percy single-handed defeats Clarisse, leader of the Ares cabin, and three others when he healed his injuries in water. This is when he learned that he is the son of Poseidon as a holographic trident appears over his head. Soon after learning that Zeus's lightning bolt has been stolen, Percy embarks on his first and eventually successful quest along with Grover and Annabeth to retrieve it.
Thalia's tree, the magical border of Camp Half-Blood, has been poisoned. Chiron is blamed for the poisoning and is dismissed from camp; Tantalus, who comes from the Fields of Punishment and cannot have food or water, takes the job. The only thing able to rid the tree of its poison is the Golden Fleece, which is located on the island of Polyphemus (another son of Poseidon), in the Sea of Monsters (the Bermuda Triangle).
Clarisse, the daughter of Ares and longtime enemy of Percy, is given the quest to go into the Sea of Monsters to bring back the Golden Fleece, which has healing powers for plants, animals, and humans. Percy and Annabeth decide to go also, not just for the Golden Fleece, but for their friend Grover who is trapped in Polyphemus' cave. Tyson, a young Cyclops whom Percy befriended at school that year, joins them on their journey. Tyson is Poseidon's son and therefore Percy's half-brother.
As they enter the Sea of Monsters, they have to get past Scylla and Charybdis. Instead of trying to sail in between the two, Clarisse automatically goes for Charybdis and Tyson supposedly dies in Clarisse's ship after it sinks. After that, Percy and Annabeth journey to many dangerous islands, and Annabeth tells Percy many things about how Thalia died and she also mentions her prophecy and tells Percy that he has a choice to make when he becomes 16 years old. Fighting their way through many other obstacles, like the Sirens and Circe's island, they join up with Clarisse, Grover, and later Tyson (who did not die), and leave with the Golden Fleece.
When they return to the mortal world, they send off Clarisse on an airplane to camp alone, and Percy, Annabeth, Grover, and Tyson are kidnapped by Luke. In a duel with him, Percy is nearly killed. He is saved by Chiron, who is proven innocent, and his relatives, the Party Ponies. When they return to camp the Fleece is put on the tree. The Fleece not only revives Thalia's tree, but Thalia too. It turns out that this was actually Kronos's plan, done so that he would have another chance to control the prophecy which governs the future of Olympus and the Olympians.
At the end Percy and Annabeth win a chariot race and Annabeth kisses Percy on the cheek and he liked it so he kissed her back.
In the third book, Grover finds two demigods at a school, Bianca and Nico di Angelo. Percy, Grover, Thalia and two of Artemis' hunters, Zoë and Bianca (who later chose to become a hunter, upsetting Nico), are sent on a quest to save Artemis, who has been kidnapped. Percy is really going to save Annabeth, who was captured while they were saving the di Angelo's.
During the journey, Percy meets a mortal girl at Hoover Dam, named Rachel Elizabeth Dare, who can see through the Mist. She helps him escape from skeletal warriors who were trying to attack Percy and his friends.