Koku is one of the protagonist 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. Koku is a young man with heterochromia iridum (his right eye is grey/black, while his left is dark teal) and long black hair. He typically wears blue shirts and white capris. Koku looks very quiet.