Prior to his departure from Soul Society, Aizen appeared as a mild-featured man with brown scholarly hair enhanced by square glasses, along with soft brown eyes. He wore the standard Shinigami uniform with a captain's haori. During his ascent to Hueco Mundo, he removed his glasses and swept his hand through his hair, revealing menacing eyes and a very different hairstyle; his hair is now swept back, with a strand hanging down his face.
In Hueco Mundo, Aizen initially wears Arrancar clothes over his shihakushō. After Orihime Inoue is taken to Las Noches, Aizen wears full clothing similar in style to the Arrancar, complete with a pink sash around the waist.
Upon his defeat by Ichigo Kurosaki, Aizen loses all of the physical changes made by his transformation and returns to the physical form he had prior to transforming.
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 those 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. However, this was just a mask to disguise his manipulative, and very dangerous, nature. In truth, he cares very little for his underlings, and is willing to use almost everyone around him as pawns to achieve his goals. He is willing to stop at nothing to further his selfish ambitions, including manipulating or killing anyone if he deems it to be necessary. He frequently toys with his underlings for amusement.
During his time as a captain, Aizen liked tofu, but he disliked boiled eggs. In his free time, he often read, and he gave a special lecture of calligraphy at the Shin'ō Academy because he excelled in this branch of arts. This optional course was immensely popular among the Shinigami students, and many of them felt great sorrow at Aizen's sudden departure.
As the commander of his army of Arrancar, Aizen controls his minions through acquired respect or fear. Ulquiorra Cifer revealed to Orihime Inoue that they are all there to further Aizen's ambitions. Aizen keeps more rowdy individuals like Grimmjow Jaegerjaquez in line through intimidation and mind games. The Arrancar Aisslinger Wernarr claims the Arrancar follow him because he has no fear, something which beings born from fear and despair find inspiring.
Gin Ichimaru has some sense of Aizen's emotions and feelings, correctly pointing out Aizen was enjoying the invasion by Ichigo and his friends and their successes within Las Noches, which Aizen himself confirmed, and noting Aizen was surprised by the arrival of Momo during the battle of Fake Karakura Town. In contrast, Kaname Tōsen has shown no such understanding, and has fallen victim to one of Aizen's more minor mind games when Aizen subtly manipulated him into punishing Grimmjow for him while Aizen himself kept up his appearance of benevolence.
Aizen's true objective was to overthrow the king of Soul Society. As he and his fellow traitor captains were departing from the Soul Society, he told 13th Division Captain Jūshirō Ukitake "no one has stood on the top of the world", not even the gods, but he aspires to "stand on the heavens and end the unbearable vacancy on the world's throne".
According to Kisuke Urahara, since mastering the Hōgyoku, Aizen has become less cautious in battle, for Aizen would not have allowed Urahara to use level 90 Kidō before he subdued the Hōgyoku. Aizen admits his new powers allow him to let his guard down. Due to this newfound power, Aizen has developed an arrogant and overconfident attitude, often not even thinking his attacks or defense through any longer like he used to, or even bothering to find out the effects of his opponents' and his own attacks. In his newest form, he has developed a severe superiority complex, believing he is now above both Shinigami and Hollows. Because of his superior attitude and high intelligence, Aizen is easily enraged when facing something or someone which he does not understand; for example, when witnessing Ichigo Kurosaki use Saigo no Getsuga Tenshō, Aizen becomes furious because he is unable to comprehend how a Human has surpassed him.