When your name has become synonymous with death and killing, one knows he is good. That is Koroshiya's problem. He's good. TOO good. And he knows it.
Koroshiya is an Elite. He is one of the best throw-down all-out bloody-knuckles fighters the Shinigami have.
Yes, did I mention he is a Shinigami?
Behind the facade of the Shinigami's role, Koroshiya is merely doing what he likes to do- he loves the violence and the death that he gets to instill. One would have to be like that to earn a name such as his- Koroshiya means killer. Belligerent, cocky, takes too many chances- this God of Death can back up his mouth with pure talent. It is that talent that keeps him employed despite his disrespect for Shinigami law.
But he has a secret. One he would kill to protect. And has.
The secret is the source of his skill, his strength, and it is in his right eye.
His right eye is where he sealed the demon.
He wanted its power but could not control it so he had to lock it away. Koroshiya very well knows it is killing him from the inside but unlike other Shinigami, his lust for power overrides any sensibilities he may once have possessed. And unlike other Shinigami, he has gained use of its demon magics.
It whispers dark thoughts, he gains its knowledge and insights, but if he were to ever slip up- even just once- the demon would slaughter him without mercy body and soul. Hidden from his peers by his leather bandanna, they have no idea what he had done nor will he let them get close enough to find out.
His viciousness is legendary, added to by the demon's own twisted nature. When he fights, the eye bleeds as if the demon in trying to claw its way out and take its revenge upon its captor.