Naiō Aizen (靄然内奥, Aizen Naiō) is a powerful and manipulative shinigami born many centuries before most of the current captains of the Gotei 13. He is also the father of Sōsuke Aizen, the man who carried on Naiō's work after his death at the hands of his dangerous offspring while in Yāolù. Despite being murdered by Aizen, Naiō found a man who agreed to allow Naiō to inhabit his body, thereby continuing to live. Since his death, he was obsessed with ruining his son's plans, although a human named Ichigo Kurosaki beat him to the punch. After discovering that a mere human had beat Aizen, Naiō has sworn to destroy Soul Society for entrusting their survival with a weakling such as a human.