Dantalion is a candidate who wishes to succeed the throne of the demon world. He's the 71st of Solomon's 72 Pillars and the Grand Duke that leads 36 armies of Hell. He first meets William when he confronts him, saying that he finally found the "one who has Solomon's soul." It's revealed in the first chapter that he has some kind of contract with King Solomon.
Illusions, mind reading, and the ability to create imitations of humans.