Aizen spent much of his career in the 5th Division,[1] eventually becoming its lieutenant under the former captain, Shinji Hirako.[2] Shinji kept Aizen under his view during this time because he felt there was something dangerous about him. 100 years ago, Aizen's experiments on Soul Reaper/hollow hybrids involved using an unknown method to cause spirits to transform into hollows, eventually culminating in the creation of the Visoreds. He framed Urahara for the hollowfication process and forced him to leave Soul Society.
During that time, Aizen met Gin Ichimaru and Kaname Tōsen, and lured them to his side. Aizen became captain of the 5th Division later on, and Ichimaru became his lieutenant and right hand man. Ichimaru and Tōsen later became captains of the 3rd and 9th Divisions, respectively, but remained loyal only to Aizen. According to Aizen, even when Hinamori was his lieutenant, he thought of no one but Gin as his subordinate.
At some point, he also saw the potential of Shuhei Hisagi, Momo Hinamori, Izuru Kira, and Renji Abarai, and took the latter three all into his division, while sending Shuhei to the 9th division under Tousen. While Kira and Hinamori were obedient and loyal, becoming the lieutenants of the 3rd and 5th Divisions under Ichimaru and Aizen, respectively, Renji proved to be too rebellious and was transferred to the 11th Division (and later to the 6th, where he too became a lieutenant).
Sometime prior to his departure from the 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.[3] Aizen appropriates Barragan's palace at Las Noches for himself and reconstructs it in his own image.
[edit] Personality
Aizen, in his new clothing after his defection, alongside Gin and Tōsen.
Aizen is a soft-spoken intellectual, accented by glasses during his time as a Soul Reaper captain. While he appears to be very kind to his subordinates, he cares very little for them in truth. According to Aizen himself, he is not bound by morals and will therefore stop at nothing to further his ambitions, including manipulating or killing anyone if he deems it necessary. 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.
Hollows follow Aizen for a variety of different reasons. Ulquiorra Cifer follows Aizen out of respect and to further Aizen's ambitions. The arrancar Iceringer states that hollows follow Aizen because he has no fear, which beings born from fear and despair find inspiring. Szayel Aporro Granz follows Aizen with the hopes that he will eradicate all non-hollows in the world. Barragan follows Aizen waiting for an opportunity to kill him. Starrk follows him because Aizen provided a way to end his loneliness through the Espada. Other hollows follow Aizen to obtain an increase in power through becoming Arrancars or out of fear, such as when Aizen threatened to kill Grimmjow Jeagerjaques if he attacked Kaname Tōsen.
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,[4] which has an unusually large concentration of spirit particles. Aside from killing the many souls used in its creation, creating the key will also vaporize most of Karakura Town.
During their ascent to Hueco Mundo, Aizen removes his glasses and sweeps a hand through his hair, revealing menacing eyes and a very different hairstyle. As he had previously revealed to Tōshirō Hitsugaya that the Aizen they all knew never existed, this would seem to be a visual representation of that claim. Before leaving, he tells Jūshirō Ukitake that no one had started out in heaven, not even the gods.[5] He then says that he will be the one to stand in the heavens and end the unbearable vacancy on the world's throne. Aizen is later shown in Hueco Mundo wearing clothing similar in style to the arrancar he has been creating; he is also seated on the throne he took from the self-proclaimed god and king of Hueco Mundo, Barragan.
