Sosuke Aizen is the former captain of the 5th Division in the Gotei 13. He later leaves Soul Societywith his followers, Gin Ichimaru and Kaname Tosen. His lieutenant was Momo Hinamori. He formerly served as the lieutenant of the 5th Division under Shinji Hirako. After waging war against Soul Society with an army of Arrancar, Aizen was defeated by Ichigo Kurosaki and sealed away by Kisuke Urahara, and then imprisoned for his crimes.
And he is reprised by Kyle Hebert in the English dub and Sho Hayami in the Japanese dub.