Aizen is a 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.
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 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 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).
When Ichigo Kurosaki and his friends invade Seireitei to rescue Rukia Kuchiki from execution, he manages to lay hands on the Hōgyoku with an elaborate plan involving Rukia's execution and the attempts to rescue her, manipulating most of the other captains of Soul Society according to his will. After obtaining the Hōgyoku, he declares his intentions openly.
The principal antagonist of the series, he resides in Hueco Mundo together with Ichimaru and Tōsen, commanding an army of Arrancar and Hollows, his ultimate goal being to overthrow Soul Society along with the Soul King and take his place as god of the world.
The Gotei 13 are the protectors of Soul Society, organizing the Shinigami into a military. Each of the 13 divisions is led by a captain, all of whom are a force to be reckoned with on any battlefield. Let's meet these Gotei captains!