Sousuke Aizen

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.