Sōsuke Aizen
Kisuke is eventually revealed to be actually the former Captain of Squad 12 after being promoted from his 3rd seat in Squad 2. He also founded the Shinigami Research and Development Institute. A century ago, Kisuke caught Sōsuke Aizen in the act of converting Hiyori and their fellow Soul Reapers into Hollows. Though Kisuke managed to save them by turning them into Vizards, learning that Aizen was after the he developed, Kisuke is forced to leave the Soul Society when framed for the crime and go into hiding in the World of the Living. Though he attempted to dispose of the Hōgyoku in the "gigai" he developed for Rukia, Aizen obtained the sphere. Later aiding the Soul Society in their battle against Aizen by creating a Fake Karakura Town, Kisuke engages Aizen in battle and is initially defeated. However, though defeated, Kisuke placed a special "kidō" seal on Aizen that activated after the Hōgyoku rejected Aizen as its master.