Amaimon

Amaimon is one of Satan's sons, the seventh-strongest of the Eight Demon Kings, and an older half-brother of Rin and Yukio Okumura. Initially indifferent to Rin, he becomes fixated on defeating him after their first clash, later attacking Rin's classmates during a live combat exam to provoke him into revealing his powers.

After his brother Mephisto Pheles sealed him away to curb his violent impulses, Amaimon is eventually released and enrolled at True Cross Academy under the guise of being Mephisto's nephew, doing little to disguise his true nature.

Calm and composed on the surface, Amaimon is in fact childish, apathetic, and almost entirely detached from human concerns, caring for little beyond his own amusement and his familiar, a hobgoblin named Behemoth. He refuses to take opponents seriously, hates losing above all else, and gradually develops an unlikely attachment to Shiemi Moriyama.