In a flashback several centuries ago, it is revealed that Kouga was part of the Kuchiki Clan as Ginrei Kuchiki's son-in-law, and was overconfident in the abilities of his zanpakuto. He helped put down a rebellion of Soul Reapers, but some grew suspicious of his power, and had him attacked. He cut down his attackers, but was framed for killing his own allies by a group of jealous high-ranking Soul Reapers. He was imprisoned as a result, but escaped with Muramasa's help. In the present, Kouga awakens, repelling Ichigo and Rukia, then suddenly stabs Muramasa. (thetvdb.com)