Koketsu, once Izou's superior, betrayed the warrior during a conflict, slicing away his right eye. This act of treachery directly caused Izou's descent into becoming a masterless ronin. Koketsu belonged to the Inferni, an organization hunting magic wands to resurrect the Dark Lord. This pursuit placed him directly against protagonists like the young elf Sonya, bearer of one such wand.

The trauma of Koketsu's betrayal left a deep mark on Izou. Years later, an injury to Izou's right shoulder violently triggered the memory, unleashing a berserk state fueled by that past treachery. Koketsu's defining actions encompass this betrayal of Izou and his dedicated service to the wand-seeking Inferni.