Sōsuke Aizen (藍染 惣右介, Aizen Sōsuke) 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 orchestrates Rukia Kuchiki's execution, fakes his own death, and slaughters the Central 46 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.