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. Heath Flick, Koku has spent the last seven years in search of his love, Yuna in hopes of reuniting with her. Quiet and nice, Koku brings red bean buns to his employer's 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.