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Sōsuke Aizen (藍染 惣右介, Aizen Sōsuke) is the former captain of the 5th Division in the Gotei 13; he later leaves Soul Society with his followers, Gin Ichimaru and Kaname Tōsen. His lieutenant was Momo Hinamori. He is the main antagonist of the series.

Despite his current villainous status and former position of authority, Aizen is a surprisingly polite, soft-spoken intellectual, often addressing his subordinates by their given names. He rarely shows any sign of alarm or distress and often has entire situations planned out well in advance. Whenever confronted, Aizen is casual and uncaring, attempting to draw out a conversation and make small talk, which often infuriates the one he is talking to.

Aizen initially appeared to be a very kind, well-respected captain and was looked up to by many, especially his Lieutenant Momo Hinamori. However, this was just a mask to disguise his manipulative and very dangerous nature. In truth, he cares very little for his underlings (both past and present), and is willing to use almost everyone around him as pawns in his twisted game of chess. In his own words, Aizen claims to be unaffected by any form of morals or ethics as he views them as mere restriction on his potential. He is, therefore, willing to stop at nothing to further his selfish ambitions, including manipulating or killing anyone if he deems it to be necessary. He is fond of long, complicated, and somewhat convoluted plots, many of which do not have any real reason until he reveals them, and frequently appears to toy with his underlings for amusement. Unlike most of the other Shinigami who appear to have some kind of honor and kindness, including some Espada, Aizen lacks genuine compassion and honor.

During his time as a captain, he was known to like tofu, but he disliked boiled eggs. In his free time, he often read and he also gave a special lecture of calligraphy at the Shinigami Academy, given that he excelled in this branch of arts. This optional course was immensely popular among the Shinigami students, and many of them felt great sorrow at Aizen's sudden departure.

As the commander of his army of Arrancar, Aizen controls his new minions either through acquired respect or fear. Ulquiorra Cifer, one of his noticeably loyal subordinates, revealed in a conversation with Orihime Inoue that they are all here to further Aizen's ambitions. For more rowdy individuals like Grimmjow Jaegerjaquez, he keeps them in line through intimidation and mind games. The Arrancar Iceringer also claims that they follow Aizen because he has no fear, something which beings born from fear and despair find inspiring. Other Arrancar occasionally follow Aizen for reasons unique to themselves, such as Szayel Aporro Granz, who follows Aizen with the hopes that he will eradicate the world of all non-Hollows.

To date, the equally manipulative Gin Ichimaru is the only person who seems to be able to see through Aizen's manipulation. Ichimaru has also shown that he has some sense of Aizen's emotions and feelings, correctly identifying that Aizen was enjoying the invasion of Ichigo and his friends and their successes within Las Noches, as well as picking-up on the fact that Aizen had registered the arrival of Hinamori in battle of Fake Karakura Town and was surprised by it. By contrast, Kaname Tōsen has shown no such understanding and has fallen victim to one of Aizen's more minor mind games when Aizen off-handedly manipulated him into punishing Grimmjow for him while Aizen himself kept up his appearance of benevolence.

Aizen's true objective is to overthrow the king of the Soul Society. As he and his fellow traitor captains were departing from the Soul Society, he told Jūshirō Ukitake that "no one has stood on the top of the world", not even the gods, but he aspires to "stand in the heavens and end the unbearable vacancy on the world's throne". To do this, he needs the Ōken (王鍵, Royal Key), a tri-pronged golden key that opens a portal to the dimension in which the Spirit King resides. While the location of the key is known only to Captain-Commander Yamamoto, Aizen knows how the original was created and how to manufacture another. To do this, 100,000 souls need to be gathered together (which will kill all the entities used) and vaporize a significant portion of the area used in its forging. His sights are therefore set on Karakura Town, which has an unusually large concentration of spirit particles.

According to Kisuke Urahara, since mastering the Hōgyoku, Aizen has become less cautious in battle; noting that before he subdued the device Aizen would not have allowed him to use level 90 Kidō. And Aizen admits that his new powers allow him to let his guard down. Also due to this newfound power, he has gotten more of a seemingly arrogant attitude, often not even thinking his attacks or defense through anymore like he used to.