Luke Castello: The Angel/Demon of Olympus:

Luke Castello was never ordinary.
He has spent every day hiding a secret from those close to him. The secret? He has three different personalities tangled up in his brain. The other two voices are completely different from his own and constantly cause trouble for him. He thought his life could never get more complicated... He was wrong.
Demons are real... An attack on his school would change everything he thought he knew. Luke still didn't understand. The demons were searching for demigods. ...