Amaimon

Also known as the "Earth King." Amaimon is supposedly the second son of Satan and another Demon Prince of Gehenna. Unlike his father and elder brother Mephisto, Amaimon had little interest in Rin after their first encounter.

He is a human-like demon. He has a spike of hair which is lightly colored compared to the rest of his dark green hair. He wears a jacket that has cross bones as buttons. He also wears a long sleeved green shirt under his jacket, which is ripped at the ends.

He appears to be calm and collected, but very confident in his powers. However he shows great anger and annoyance when someone gets in the way of his fun. He is very playful and doesn't take things seriously when it comes to fighting strong combatants which hints that he is very skilled in combat.

(Source: Ao No Exorcist Wikia)