|Affiliation||Himself, His Arrancar Army|
|Previous Affiliation||Gotei 13, Soul Society|
|Position||Leader of the Arrancar Army|
|Previous Position||Captain of the 5th Division|
Lieutenant of the 5th Division
Instructor at the Shin'o Academy
|Previous Division||5th Division|
|Partner||Gin Ichimaru, Kaname Tosen|
|Previous Partner||Momo Hinamori, Shinji Hirako|
|Base of Operations||Las Noches|
- "All creatures place their faith in someone more powerful than them and they cannot survive unless they blindly follow that person. The recipient of that faith then seeks out someone in an even higher position in order to escape that pressure. That person then seeks out someone even more powerful that he must put his faith in. In this way, all kings are born and in this way, all Gods are born."
- ―Sosuke Aizen[src]
Sosuke Aizen is a Shinigami and was once the Captain of the 5th Division in the Gotei 13 until he betrayed them in the name of seeking power and created an army of Arrancar to conquer the Soul Society.
Aizen is a tall man with brown eyes and brown hair that is swept back with a strand hanging in his face. He wears a similar uniform to the Arrancar under his command. He is famous for having a charming smile that can be compared to "gazing into the Sun", though this hides his true villainous nature.
At first glance, Aizen seems to be a calm, formal, and impeccably polite man who at first glance seems to be kind and charismatic. However, this is only a facade. Underneath his signature charming smile and sophisticated demeanor lies a callous, sadistic, egomaniacal, and power-hungry Satan-in-sheep's-clothing who will stoop to anything to get what he wants. He usually rules his underlings through his remarkable charisma and charm, but will sometimes use intimidation to keep his more rowdy soldiers (such as Grimmjow Jaegerjaquez and Baraggan Luisenbarn) in line. Aizen doesn't care about any of his underlings, viewing them as pawns and tolerating their existence only as long as they can be useful to him.
Whereas some villains (even members of the Espada) show some level of honor, Aizen doesn't have any and freely declares this, calling honor a weakness and stating that hatred is necessary for survival. Aizen maintains a calm visage that rarely, if ever slips. This gradually slips away, however, as he shows signs of increased mental instability as his power increases, shedding any signs of supposed caution and sophistication. By the time he reaches the pinnacle of his power, Aizen loses all traces of composure, laughing insanely when he thinks he has the upper hand and going into fits of manic, even childish, rage when his enemies get the better of him.
For all his supposed power and intelligence, Aizen is a coward at heart. In both timelines, he relies heavily on using his Shikai to defeat enemies whose overall skills surpass his own, and he also has a tendency to over-complicate things, such as the function and power of the Hogyoku, largely to satisfy his enormous ego. His most prominent flaw, in both timelines, is his arrogance. Despite claiming to take precautions against those who may pose a genuine threat to him, at no point does he even consider the possibility that his plans will fail, ultimately relying on luck as his ploys fall apart one by one. This, coupled with his habit of underestimating his enemies, would ultimately prove (in both timelines) to be his eventual undoing.
Aizen's exact origins are unknown, but at some point he joined the Gotei 13 and became the lieutenant of Captain Shinji Hirako of the 5th Division. During his tenur as a lieutenant, he met a young Gin Ichimaru and, intrigued by the boy's talent, took him under is wing, though Gin secretly hated him and planned to kill him for hurting his friend, Rangiku Matsumoto. He also manipulated 9th Division lieutenant Kaname Tosen into serving him by appealing to the latter's sense of "justice".
Aizen made his first step into true villainy when he performed a Hollowfication experiment on eight Shinigami, including Shinji, and framed Kisuke Urahara, then captain of the 12th Division, for the crime. The nine were marked for death, but escaped with the help of 2nd Division captain Yoruichi Shihōin, who was also declared a criminal, and fled into exile. Later, Aizen replaced Shinji as captain of the 5th Division, while Gin and Tosen became the captains of the 3rd Division and 9th Division, respectively.
Aizen manipulated the Gotei 13 into lowering their guard by portraying himself as a benevolent, charismatic captain who was supposedly very close to his lieutenant, Momo Hinamori, who practically worshiped him. He placed them all under the spell of his Shikai, Kyoka Suigetsu, allowing him to manipulate their senses and further making them let their guard down. He also secretly began using various methods to build an army of powerful Arrancar, the strongest of which became the Espada, to destroy the Soul Society when the time was right.
Aizen showed his true colors when Ichigo Kurosaki and his friends invaded the Soul Society to rescue seated officer Rukia Kuchiki, who had been condemned to death through Aizen's manipulations--Ichigo's meeting with Rukia and various other events of his life had also been orchestrated by Aizen, as it turned out. After orchestrating a series of confusing events, including staging his own death and framing 10th Division captain Toshiro Hitsugaya for it; massacring the Central 46, the Soul Society's corrupt judicial branch; and weakening several of his enemies through various battles, Aizen revealed his treachery to his former comrades.
He stole the Hogyoku - a sphere that could help him increase his power - by removing it from Rukia's body, and escaped to Hueco Mundo, specifically Las Noches, with Gin and Tosen, declaring that he would kill the Soul King and rule the Soul Society. As a way to further symbolize his betrayal, Aizen destroyed the glasses he had always worn as a captain and swept a strand of his hair back, revealing a more menacing appearance. He bid his former comrades farewell before disappearing into the garganta.
- Aizen possesses one of the strangest Zanpakuto in the series due to its illusory powers.
- During his time in the Gotei 13, Aizen wore glasses, but since he destroyed them after revealing his deceit, it is unlikely that he is truly near-sighted.
- Aizen is the main antagonist of the first volume in both timelines.
- In both timelines, Aizen is killed by a supposed ally after being stripped of his powers.