Percy Jackson is a demigod, the son of the mortal Sally Jackson and the Greek god Poseidon. He has ADHD and dyslexia, allegedly because he is hardwired to read Ancient Greek and has inborn "battlefield reflexes". He has been expelled from almost every school he has ever attended. Percy's birthday is August 18.[1] In the first novel of the series, The Lightning Thief, he is twelve years old. Like other demigods, he is expected to act as the Olympian gods' agent in the mortal world, taking action when ancient laws forbid direct godly intervention, usually in the form of quests. His personality is described as "changeable like the sea" (a quality he shares with his father) and hard to predict—with the important exception that he is dangerously loyal to his friends and family. He also displays deep regret when he sees others around him being hurt, often even monsters and strangers. His "powers", which start small and develop as the books proceed, include controlling water, making hurricanes, breathing underwater, and talking to horse-like animals and fish. He also becomes an accomplished sword-fighter and leader, as well as being the head counselor of the Poseidon cabin at his demigod summer camp.

Percy Jackson's fatal flaw is loyalty, which he at first does not see as a weakness. Throughout Percy Jackson & The Olympians, Percy grows more confident and courageous.[2] After The Last Olympian, the next time Percy is seen is in The Son of Neptune, the second book in the sequel series The Heroes of Olympus. He is suffering from amnesia and slowly struggles to regain his memory over the course of the novel. During the remaining three books in the series, previously unseen or nonexistent facets of his character are revealed—due in part to the series being told in the third person rather than the first. Some of these include a new fear of drowning; the humbling realization that he has become lax and almost cocky in his training; and a tendency to take things too far in defense of his friends and family. One of the darkest moments in the novels comes in The House of Hades when Percy discovers that he can control the water in a person's (specifically the misery goddess Akhlys's) body.[3] Luckily for him, his girlfriend Annabeth Chase is present and brings him to his senses—showing yet again his loyalty, but also how his moral character remains mostly intact despite all the difficult situations he has been forced to deal with.

At the end of the last book in the series, Percy and Annabeth Chase (his girlfriend) plan to finish their senior year of high school together in New York, and then attend college in New Rome (the demigod-only city located in California).