Sousuke Aizen is a fictional character from Tite Kubo's manga and anime Bleach. He begins as the gentle, bespectacled captain of the Fifth Division of the Gotei 13, the Soul Reaper force of Soul Society, before he is revealed as the main antagonist of the first major arc. He is voiced in Japanese by Show Hayami and in English by Kyle Hebert.

He is known for manipulation. He frames Kisuke Urahara, fakes his own death, and later builds an army of Arrancar in Hueco Mundo because he wants to overthrow the Soul King and take the throne of the heavens. He believes that throne stands empty and that he should be the one to fill it. He is defeated by Ichigo Kurosaki and sealed away, though he is briefly freed during the Quincy Blood War.

The people who end up texting him tend to enjoy slow, clever conversation and don't mind being looked down on with polite condescension. He treats most people as pieces to be moved, so expect dry wit, long philosophical asides, and cool composure rather than warmth.