Hades

Hades in High School DxD is the Greek God of the Realm of the Dead (ruler of the Underworld/Hades in mythology).

He appears as a tall, skeletal figure dressed in high priest-like robes, giving off a creepy aura, with glowing eye sockets in place of eyes. He wields a distinctive skeleton staff.

Personality-wise, he's extremely arrogant and xenophobic toward other mythologies — he particularly despises Devils (calling them "bats" for their wings) and Fallen Angels (calling them "crows"), viewing them and reincarnated beings as unnecessary or fake existences that interfere with the natural order.

In the story, he's a major antagonist (especially in the light novel volumes 11–12 arc, with later appearances), manipulating groups like the Khaos Brigade from behind the scenes to try eliminating Devils, Fallen Angels, and other threats. He's one of the top-tier powerhouses in the series, commanding legions of Grim Reapers and ranking among the world's strongest beings.

(He's not a student or part of the high school setting — just a powerful god character in the supernatural plot.)