Sosuke Aizen.

Synopsis;

Aizen spent much of his career in the 5th Division, eventually becoming its Lieutenant under the former Captain, Shinji Hirako. Shinji kept Aizen under his observation because he felt there was something dangerous about him; dark and brooding. 100 years ago, Aizen experimented on Shinigami / Hollow integration involving Hollowfication to cause spirits to transform into Hollow. He then framed Urahara for the Hollowification process and forced him to leave Soul Society.

During that time, Aizen met Gin Ichimaru and Kaname Tosen, 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 Tosen later became Captains of the 3rd and 9th Divisions, respectively, but remained loyal only to Aizen. By his own admittance, even when Hinamori was his Lieutenant, he thought of no one but Gin as his true subordinate.

At some point, he also saw the potential of Momo Hinamori, Izuru Kira, and Renji Abarai, and took them all into his division. 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).

While he appears to be very kind to his subordinates during his time as Captain, he cares very little for them in truth. According to Aizen himself during his split from Seireitei, 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.

During his ascent to Hueco Mundo, Aizen removes his glasses and sweeps a hand through his hair, revealing menacing eyes and a very different hairstyle which seems to portray his "new" identity. As he had previously revealed to Renji Abarai that the Aizen they all knew never existed, this would seem to be a visual representation of that claim. Before he leaves, he tells Jushiro Ukitake that no one had started out in heaven, not even God. 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 a throne, implying that he has the highest level of political power within the hierarchy.

It has been conjectured that Aizen's true objective is to overthrow the King of Soul Society- a God which seems blind to the eternal struggles. To do this, he needs the royal key, 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. Aside from killing the many souls used in its creation, creating the key will also vaporize most of Karakura Town.