Kokuu

黒羽

Following the attack on Jaula Blana orchestrated by Gilbert Ross, which killed several of his classmates including Dr. Health Flick, Koku has spent the last seven years searching for his love, Yuna, hoping to reunite with her.
Koku has heterochromia iridum and long black hair. Usually dressed in blue shirts and white capris, he appears very quiet and observant.
He is also the black winged king. In this form, his left eye turns blue-green and his skin turns black. Two black wings sprout from his back. Most notably, his left arm turns into blue steel on command and serves as a weapon. His left leg, taken from Izanami, can also turn into steel on command and is used as a weapon.
Quiet and kind, Koku brings red bean buns to his employer's family. He is skilled at repairing violins, though he does not play them himself.
His secret persona, Killer B, has a different personality. This side is more aggressive and solely focused on his goals. He shows no mercy, killing all his enemies and even seemingly ordinary criminals.
The kanji in Koku's name are black (黒) and feather (羽).