Sousuke Aizen (藍染 惣右介, Aizen Sousuke) is the former captain of the 5th Division of the Gotei 13, whose lieutenant was Momo Hinamori; before his captaincy, Aizen served as the division's lieutenant under Shinji Hirako.

For over a century, Aizen masqueraded as a benevolent Shinigami officer while secretly plotting his betrayal and conducting depraved experiments, which around 100 years ago led to the Hollowfication of the Visored and their (alongside Kisuke Urahara's) forced exile. Aizen's plans reach a tipping point after he slaughters the Central 46, orchestrates Rukia Kuchiki's execution, and fakes his own death in a successful ploy to obtain Urahara's Hōgyoku. After retrieving said artifact and revealing his treachery, Aizen and his longtime followers, Gin Ichimaru and Kaname Tōsen, retreat from Soul Society into Hueco Mundo, where Aizen is revealed to have built an army of Arrancar.

For his ultimate goal of reaching and overthrowing the Soul King, Aizen uses his forces to wage war against the Gotei 13. After prevailing over most of the Shinigami in his way and achieving a transcendent state through his fusion with the Hōgyoku, Aizen is nonetheless defeated by Ichigo Kurosaki, sealed by Urahara, and later imprisoned in Muken for his crimes. During the Quincy Blood War, Aizen is temporarily freed by Shunsui Kyōraku to help fight against the Quincy King, Yhwach, and plays a pivotal role in the latter's defeat.