Koku is one of B: The Beginning's main characters, working as a violin craftsman under Lily's father while harboring a massive secret: he's the serial killer known as Killer B, named for the distinctive way he marks crime scenes with the number "13."
Seven years before the series begins, an attack on Jaula Blana orchestrated by Gilbert Ross killed several of his classmates, including Dr. Health Flick. Since then, Koku has been searching obsessively for his love, Yuna, hoping to find her again.
In his everyday life, Koku comes across as quiet and observant. He has heterochromia iridum and wears his hair long and black, typically favoring blue shirts and white capris. He's genuinely skilled at violin repair—though he doesn't actually play—and he's considerate enough to bring red bean buns to his employer's family. Pretty normal guy, basically.
But Koku is also the black-winged king, capable of transforming into a heteromorphic being. When this happens, his left eye shifts to blue-green, his skin turns black, and two massive black wings sprout from his back. His left arm transforms into blue steel that he can weaponize on command. His left leg, taken from Izanami, works similarly.
His Killer B persona is a different beast entirely—aggressive, merciless, and laser-focused on his goals. Where human Koku is gentle, Killer B shows no mercy to anyone in his way, whether they're actual enemies or just common criminals caught in the crossfire.
Content compiled by AnimeList.moe from publicly available sources.
