Demetri is a valued member of the Volturi guard. He has black, shoulder-length hair and his eyes are the same as most vampires - red. He has an olive complexion that looks odd combined with the normal vampiric pallor. Demetri's gift makes him the most talented tracker in the world, Edward describes it like this in the novel New Moon: He catches the...tenor...of someone's mind, and then he follows that. It works over immense distances. This is why Aro, the leader of the Volturi, keeps him.