Sōsuke Aizen (藍染 惣右介, Aizen Sōsuke?) is a fictional character and major figure in the anime and manga series Bleach created by Tite Kubo. He is the captain of the 5th Division when first introduced, but then he betrays Soul Society and becomes the primary antagonist of the series. His former lieutenant is Momo Hinamori.

Source: Wikipedia