Kokuu  

Black-Winged King, 黒羽

For the past seven years, following the attack on Jaula Blana led by Gilbert Ross that resulted in the deaths of several classmates including Dr. Health Flick, Koku has been searching for his love, Yuna, with the hope of reuniting with her. Koku is characterized by heterochromia iridum and long black hair. He usually wears blue shirts and white capris, giving him a quiet and observant appearance. In addition to being the black-winged king, Koku's left eye turns into a shade of blue-green when activated, while his skin becomes black. Two black wings sprout from his back, and his left arm transforms into blue steel on command, which he uses as a weapon. His left leg, obtained from Izanami, can also turn into steel on demand and is used for combat purposes. Koku is known to be polite and bring red bean buns to his employer's family. He has skills in violin repair but does not play them himself. However, when he adopts the persona of Killer B, his demeanor changes significantly, becoming more aggressive and focused solely on achieving his goals. Enemies and seemingly common criminals do not receive mercy from him. The kanji in Koku's name are black (黒) and feather (羽).