Kaji is the younger (adopted) sister and former Tsuguko of the former Flame Hashira, Kyojuro Rengoku. She became Kyojuro's successor after his sudden death, a reason to her cold and stoic attitude which is the complete opposite to her late brother’s attitude. As a Hashira of the Demon Slayer Corps, Kaji is one of the most powerful and skilled swordsmen in the entire organization. As the Flame Hashira, Kaji is his generation's most skilled Flame Breathing user. Kaji still mourns Rengoku’s death.