Percy Jackson is the 12 year old half Brother of King Triton. He's Half Atlantean and Half Human. The only one of his kind. Which is why Cadmus took such interest in him.
At age 8, Percy was kidnapped by the light and brought to Cadmus. In order to get to him though, they had to kill his mother.
Four years later, Percy is able to escape when Robin, KF, and Aqualad come to Cadmus. He then flees to Gotham, where he then runs into Batman and the Boy Wonder.
After a not-so-interrogation from the Justice League, Percy finds himself at the home of an old friend, Artemis Crock. He knows her past and she knows his, and their brother/sister bond is unbreakable because of it.
Artemis is then introduced to the YJ team and Percy secretly tags along on their first mission.
Ship: Loner Percy at beginning with SuperboyXPercy fluff, then a SuperboyXPercy relationship.
Disclaimer: I do not own PJO or YJ or JLA