Kōga Kuchiki

is a rogue Shinigami who once revolted against the Seireitei.

History
Sometime before the main storyline, Kōga revolted against the Seireitei. He used his Zanpakutō, Muramasa, to make other Zanpakutō spirits abandon their wielders and attack them. The Shinigami were eventually able to seal Kōga away. Kōga's revolt was never recorded in the Soul Society's history.

Synopsis
Kōga was sealed away, but Muramasa eventually resurfaces and begins to initiate a Zanpakutō rebellion. When Ichigo Kurosaki encounters Muramasa, he states that he already killed his master. However, after Muramasa gains control over Ryūjin Jakka, Genryūsai Shigekuni Yamamoto reveals that Muramasa is planning to break Kōga's seal. He also states that Muramasa is heading to Karakura Town, where Kōga was sealed.