Sōsuke Aizen is a fictional character from the manga series Bleach created by Tite Kubo and the main villain of the first half of the series (and the most significant one for the entirety of the anime adaptation). He is the captain of the Fifth Division of Soul Reapers when he is first introduced. He later betrays the Soul Society, before Ichigo Kurosaki and Kisuke Urahara defeat him. His former lieutenant is Momo Hinamori. Aizen spent his life working at the Fifth Division, eventually becoming its lieutenant under the former captain, Shinji Hirako. Aizen gained followers, including Gin Ichimaru and Kaname Tōsen. A century before the series’ setting, after Kisuke Urahara created the Hōgyoku, an item that he believed creates Soul Reaper/Hollow hybrids, Aizen began to experiment using the Soul Reapers that became the Vizards. Among his test subjects was Shinji, who knew about Aizen, but learned too late that Aizen used his mistrust against him.