Percy Jackson and the Titan's Curse

Percy Jackson has had adventures since discovering that he was the son of Poseidon, but now things get even more dangerous. After Thalia became a human again, they are sent to a military school where there are two demigods. Percy's mom drives them there.  When they get there they notice something very odd about one of the co-principals. One of his eyes is brown and the other is blue. What is he? You can find out easily if you read Percy Jackson and The Titan's Curse.