Koku is one of the main characters in B: The Beginning. He works as a Violin Craftsman under Lily's father and has the ability to transform into a black-winged, heteromorphic being. He is also the true identity of Killer B, a name derived by his unusual way of writing the number "13" at his murder sites.
Since the attack on Jaula Blana orchestrated by Gilbert Ross that took the lives of several of his classmates including Dr. Health Flick, Koku has spent the last seven years in search of his love, Yuna in hopes of reuniting with her.

Koku has heterochromia iridum, and has long black hair. Typically wearing his blue shirts and white capris, Koku looks very quiet and observant.

He is also the black winged king. In that case, his left eye turns into a blue green and his skin black. Two black wings sprout from his back. Most notably, his left arm turns into blue steel on command and is used as a weapon. His left leg, taken from Izanami, is able to turn into steel on command, and is also used as a weapon.

Quiet and nice, Koku brings red bean buns to his employers family. He is skilled at fixing violins but doesn't play them himself.

His secret self as Killer B has a different personality. This personality is more aggressive, solely focused on his goals. He doesn't show mercy as he kills all his enemies and even seemingly common criminals.

The kanji in Koku's name are black (黒) and feather (羽).

(Source b-the-beginning Wikia)