Aizen spent much of his career in the 5th Division,eventually becoming its lieutenant under the former captain, Shinji Hirako. Shinji kept an eye on Aizen during this time because he felt there was something dangerous about him; his distrust valid yet was ultimately exploited by Aizen so he can carry out his agenda in secret with Shinji unaware until it was too late. One hundred years before the series' setting, Aizen's experiments on Soul Reaper/hollow hybrids. Aizen framed Urahara for the hollowfication process and forced him to leave Soul Society.
Sometime prior to his departure from Soul Society, Aizen, along with Tōsen and Gin was shown to have travelled to Hueco Mundo, arranging an alliance with the self-proclaimed king and god of Hueco Mundo, Barragan Luisenbarn. Aizen appropriates Barragan's palace at Las Noches for himself and reconstructs it in his own image, forcing Barragan into his service.
While he appears to treat his subordinates with kindness and compassion, he views them as little more than pawns in reality. By his own admission, he views morality and empathy as weaknesses which must be expunged in order to reach one's full potential. Consequently, he thinks nothing of manipulating or killing those around him in order to achieve his goals as he has enough information to manipulate them emotionally as well. But after fusing the Hōgyoku, Aizen becomes so confident that he doesn't bother to plan against attacks, thinking himself to be invincible, which ultimately leads him to his defeat at the hands of Ichigo and Urahara, and the destruction of his zanpakuto, Kyoka Suigetsu. He also discards his calm nature, becoming enraged when Ichigo Kurosaki dominates him beyond what he could comprehend. Tite Kubo stated in an interview that "part of Aizen's appeal is that he's mysterious," and for this reason he didn't think it's a good idea to go too much in-depth with the character. During his tenure as a Soul Reaper, Aizen's public image was that of a bespectacled intellectual whose humility and warmth was equalled only by his dedication to the Gotei 13. However, it eventually became apparent that this was a carefully crafted facade designed to draw suspicion away from his true, sinister nature.
It has been conjectured that Aizen's true objective is to overthrow the king of Soul Society. To do this, he needs the royal key (王鍵ōken?), a tri-pronged golden key that opens a portal to the dimension that the king resides in. While the location of the key is known only to General Yamamoto, Aizen knows how the original was created and how to manufacture another. In order to create the key, Aizen needs to gather 100,000 souls in Karakura Town, which has an unusually large concentration of spirit particles.
Aizen's zanpakutō is Kyōka Suigetsu (鏡花水月?, literally "mirror flower water moon"). It is activated with the command "shatter" (砕けろkudakero?). Kyōka Suigetsu's Shikai ability is Kanzen Saimin (完全催眠, Complete Hypnosis). It controls the five senses to the point that it can make the target misinterpret form, shape, mass, feel and smell. The initiation condition for the hypnosis is to show the enemy the release of Kyōka Suigetsu. After seeing it just once, that person will completely succumb to the hypnosis. After that, every time it is released, that person will fall under the thrall of the hypnosis. The hypnosis is apparently long-lasting, seeing as the Visoreds are still affected by Kyōka Suigetsu even though they were hypnotized at least 110 years ago. Since the spell only works upon seeing the ritual, those who cannot see are consequently immune to the hypnosis. When Aizen turns off the hypnosis by force of will, the image shown melts away/shatters. Kyōka Suigetsu's Kanzen Saimin technique is absolutely flawless; even if the target is aware that they are under the hypnosis, they cannot resist its influence. Kyōka Suigetsu's hypnotic abilities make it a highly useful tool in combat, as Aizen can quickly create simple or complex illusions to deceive opponents. For example, by creating a decoy of himself, or other illusions to distract his opponents, Aizen can hide his own movements and attack with impunity. His opponent, meanwhile, will remain focused on the illusion until it's too late to react.Aizen is an extremely powerful combatant, well above many captain-level Soul Reapers. When preparing to activate the Hōgyoku, he states that despite its half-awakened state, it can be fully activated temporarily when fusing with someone with at least twice as much spirit pressure as an average captain-level Soul Reaper, referring to himself.
After Aizen embedded the Hōgyoku into his body, he undergoes a series of transformation that granted him extraordinary boost in both speed and strength.