Percy is the son of Poseidon, called Neptune in The Heroes of Olympus. He begins in The Lightning Thief as a troubled twelve-year-old boy with dyslexia and ADHD. When his class takes a field trip to the Metropolitan Museum of Art, one of his teachers turns into a Fury and attacks him. His friend Grover then reveals himself to be a satyr and takes Percy to Camp Half-Blood, a place were demigods can train. Percy then learns that he, being a child of Poseidon, is mentioned in the Great Prophecy given by the Oracle. With the help of the other demigods at Camp Half-Blood, Percy defeats the forces of Kronos and saves the camp. He is then given a new prophecy by the next Oracle, his friend Rachel Elizabeth Dare.
In The Heroes of Olympus, Percy begins the series missing, with Annabeth Chase and his other friends searching for him. He returns with amnesia in The Son of Neptune, where he comes across Camp Jupiter, a camp for the children of the Roman gods. Mars tells him to go with other demigods to Alaska and free the god of death, Thanatos. He regains his memory, and he and Annabeth travel through Tartarus in an attempt to defeat Gaia's forces and close the Doors of Death.