Kaigaku

While alive, Kaigaku was a sturdily built young man of medium height with short, spiky black hair, thick arched eyebrows, and cold teal eyes locked in an almost permanent sneer. Turning demon gave him flame-shaped marks on his cheeks, fangs, pallid skin, blackened nails, and emerald irises set in black sclera, later inscribed with his Upper Rank Six insignia. Brash on the surface yet cowardly and self-serving beneath, he was an orphan who would resort to anything to stay alive, even sacrificing the children in Gyomei's care to spare himself. He carried a hypocritical sense of righteousness, scorning Zenitsu as worthless despite sharing the very same limitation.

Kaigaku trained alongside Zenitsu under their teacher, the retired Thunder Hashira Jigoro. He learned every Thunder form except the first, the lone technique Zenitsu could pull off, and bristled at being named joint heir with him. During the Hashira Training period a mission put him in front of Upper Rank One Kokushibo, and he gave up at once to save his skin, becoming a demon and filling the Upper Rank Six seat left empty by the slain Daki and Gyutaro.

His treachery pushes Jigoro to seppuku, and Zenitsu hunts him for revenge. The two confront each other within the Infinity Castle, where Kaigaku coldly brushes off their master's death as well earned. Zenitsu condemns his hypocrisy, pointing out that neither of them ever truly mastered Thunder Breathing, before cutting him down with one decisive blow.

Kaigaku uses Thunder Breathing, having mastered all forms but the first, and as a demon he gained an Electrokinesis Blood Demon Art that runs lightning through a flesh blade shaped after his old Nichirin sword. As an Upper Rank he commanded immense speed, strength, and regeneration. His fatal flaw mirrored his rival's: lacking the first and most basic Thunder form, he was undone by the single technique Zenitsu had perfected.