Despite his current villainous status and former position of authority, Aizen-sama is a surprisingly 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-sama is casual and uncaring, attempting to draw out a conversion and make small talk, which often infuriates the one he is talking to.
Aizen-sama initially appeared to be a very kind, well-respected captain and was looked up to by many. However, this was just a mask to disguise his manipulative and very dangerous nature. In truth, he cares very little for his underlings (both past and present), and is willing to use almost everyone around him as pawns in his twisted game of "chess". In his own words, Aizen-sama claims to be unaffected by any form of morals or ethics as he views them as mere restriction on his potential. He is, therefore, willing to stop at nothing to further his ambitions, including manipulating or killing anyone if he deems necessary. He is fond of long, complicated, and somewhat convoluted plots, many of which do not have any real reason until he reveals them, and frequently appears to toy with his underlings for amusement.
During his time as a captain, he was known to like tofu, but he disliked boiled eggs. In his free time, he often read and he also gave a special lecture of calligraphy at the Shinigami Academy, given that 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-sama's "sudden departure".
As the commander of his army of Arrancar, Aizen-sama controls his new minions either through acquired respect or fear. Ulquiorra Cifer, one of his noticeably loyal subordinates, revealed in a conversation with Orihime Inoue that they are all "here to further Aizen-sama's ambitions". For more rowdy individuals like Grimmjow Jaegerjaquez, he keeps them in line through intimidation and mind games. The Arrancar also claims that they follow Aizen-sama because he has no fear, something which beings born from fear and despair find inspiring. Other Arrancar occasionally follow Aizen-sama for reasons unique to themselves, such as Szayel Aporro Granz, who follows Aizen-sama with the hopes that he will eradicate the world of all non-Hollows.
To date, the equally manipulative Gin Ichimaru is the only person who seems to be able to see through Aizen-sama's manipulation. By contrast, Kaname Tōsen has shown no such understanding.
Aizen-sama's true objective is to overthrow the king of the Soul Society. As he and his fellow traitor captains departed the Soul Society, he told Juushirou Ukitake that "no one has stood on the top of the world", not even the gods, but he aspires to "stand in the heavens and end the unbearable vacancy on the world's throne". To do this, he needs the Royal Key (王鍵, ōken), a tri-pronged golden key that opens a portal to the dimension in which the Soul King resides. While the location of the key is known only to Captain-Commander Yamamoto, Aizen-sama knows how the original was created and how to manufacture another. To do this, 100,000 souls need to be gathered together (which will kill all the entities used) and vaporize a significant portion of the area used in its forging. His sights are therefore set on Karakura Town, which has an unusually large concentration of spirit particles
High Intellect: Arguably the most fearsome trait of Aizen-sama is his tremendous intellect. He is well-versed in the history, strategy, tactics of the Soul Society, and is privy to knowledge previously unknown to many, such as knowing of the existence of the ōken, as well as the obscure method in which to make one. He has also created several hollows and Arrancar while being able to avoid arousing any suspicion from the majority of his fellow Shinigami, most likely due to his Zanpakutō's illusionary abilities. He also, for over 110 years, had been engaging in experiments that involved unique and highly dangerous manipulations of spirit energy and spirit particles. His most startling experiment involves the Hollowfication of Shinigami, something unseen before and still unknown to have ever taken place again. He has an innate knowledge of the Hōgyoku, rivaling that of its creator, Kisuke Urahara, even the extent required for him to destroy it. He is able to figure out Shinji Hirako's shikai ability's true nature and then deduce that it produces an optical illusion, allowing him to counter its effect.
Master Manipulator: Throughout the series, he has shown himself to be a very crafty and cunning man since his public betrayal of the Soul Society. The most noticeable trait is his ability to deceive and manipulate others around them in multiple manners. For years, he was effectively able to convince everyone around him that he was a kind-hearted man with the best intentions for the Soul Society, while still performing horrific actions. He has a great talent for analyzing and observing everything in his presence, allowing him to quickly understand a person's pattern of thinking and better manipulate them. He is also a powerful strategist, capable of formulating well-thought-out plans several steps ahead of others. These plans are incredibly involved and detailed, and are planned-out with the long term in mind, making them seem slightly convoluted. His most well known plan spans over 110 years, a great accomplishment given that it required moving people around like pieces on a chessboard, as well as making sure they played the role he had planned out for them. He is masterful when it comes to "plans within plans," such as kidnapping Orihime Inoue, allowing him to use her abilities to extend the shelf life of the Hōgyoku, and to trap the Ryōka and 4 Captains after the departure of Gin, Tōsen and himself in Hueco Mundo, making it easier for him to accomplish the ends of his first plan, to create the ōken and gain access to the Royal Dimension.
Master Tactician: He has also shown himself as a powerful and flexible tactician, capable of adjusting seamlessly to any given combat situation. Though it can be said that the majority of his tactics rely heavily on his Zanpakutō's special ability, there are few better tactics than manipulating all the senses of your opponents, making it easier for one to attack. Despite viewing his power as worlds beyond others, he is also shown to be highly cautious in battle, placing special protections on himself to leave no blind-spots.
Aizen-sama using Hadō #90: Kurohitsugi (Black Coffin).
Kidō Master: Aizen-sama has a great mastery at using Kidō spells. He is skilled enough to use Kidō without the use of their incantation; he used the Black Coffin (ranked 90) spell, and even though he did not control it entirely (utilizing only a third of the spell's original strength), it was sufficient enough to disable Captain of the Seventh Division, Sajin Komamura. He has even used the Splitting Void (ranked 81) spell to block another Kidō attack by Tessai Tsukabishi, who was Captain of the Kidō Corps and reasonably the most proficient Kidō master in the Soul Society at the time, while Aizen-sama himself was only a lieutenant. He is also shown able to produce powerful continuous spells to act as extra protection to his being while in battle.
Immense Strength: He is powerful enough to stop Ichigo's Bankai with just one index finger (Ichigo was already battle-worn from fighting Byakuya Kuchiki) and almost cut him in half with a single swing of a sword. He also stopped Renji Abarai's Shikai with his bare hand and broke it with one swing, injuring Renji in the process. He was even able to stop Sajin Komamura's Bankai blade with his bare hand, proving his strength was more than enough to combat someone of his same level. Aizen-sama further shows his strength by being able to effortlessly cut through the sword and wrist of Komamura's Bankai.
Immense Spiritual Power: Even by captain-level standards, Aizen-sama has shown to have tremendous spiritual energy. Marechiyo Ōmaeda commented that Aizen-sama's very presence is monstrously overwhelming. A hint to his actual level of spirit power is given when he turns Wonderweiss Margera into an Arrancar; before activating the Hōgyoku, and states that despite its half-awakened state, it can be fully activated when temporarily fusing with someone with twice as much spiritual power (referring to himself) as an average captain-level Shinigami. The force of Aizen-sama's spiritual power is also great enough to, without any visible effort, bring the 6th Espada, Grimmjow Jaegerjaquez, to his knees. He has commented that it takes a lot for him not to "crush an ant" (referring to Renji) in his path. He also claims that he is more powerful than all of the Espada. Aizen-sama has shown that his reiatsu is powerful enough to crush any of Soi Fon's attacks and powers by negating her Nigeki Kessatsu technique.
Master Swordsmanship Specialist: While the full extent of his prowess has yet to be known, he has demonstrated the ability to effortlessly fight back an opponent's Shikai with his own blade still sealed. He has also demonstrated the ability to perform tremendously powerful and precise strikes, capable of striking through very sturdy targets.
Flash Steps Master: He is perceived to be so fast that one cannot discern even being attacked by him until after it has taken place. Though it's questionable if he is as fast as Yoruichi Shihōin or Soi Fon, especially seeing as they both caught him before he was able to unsheathe his sword to engage in battle before his escape from the Soul Society. Aizen-sama is shown to easily outmaneuver Ichigo's Bankai and mask-enhanced speed to the point he could casually lay a hand on his chest and still dodge a point-blank attack. He is capable of effortlessly dodging simultaneous attacks at close range and the evasion of Bankai level techniques. He moves so fast that opponents commonly do not know his movements until after he has made them and is skilled enough in his movements to evade at high speeds even when being attacked from behind.
Kyouka Suigetsu (鏡花水月, Mirror Flower, Water Moon): In its sealed form, it resembles a normal katana. The only decoration is the crossguard, which is a hexagon instead of a rectangle. The two sides parallel to the blade are long, whereas the four sides above and below the blade are shortened, making it into almost a diamond or prism-like shape. It has a green handle.
* Shikai: The release command is "Shatter" (砕けろ, kudakero) and is used to both activate and deactivate it.The name Kyouka Suigetsu hints at its illusionary properties; the moon reflected in water and a flower in a mirror are things that can be seen but not held. It is a Chinese saying for seeing and desiring something that is a dream that cannot be easily grasped in hand, teaching people about temptations.
Shikai Special Ability: Kyouka Suigetsu's Shikai ability is Complete Hypnosis (完全催眠, kanzen saimin). It controls the five senses to the point that it can make the target misinterpret another person's form, shape, mass, feel and smell to be that of the enemy. The initiation condition for the hypnosis is to show the enemy the release of Kyouka Suigetsu. After seeing it just once, that person will completely succumb to the hypnosis. After that, every time it is released, that person will become under Aizen-sama's control. Since the spell works when you see the ritual, those who can not see are consequently immune to the hypnosis. When he turns off the hypnosis by force of will, the image shown melts away/shatters. Kyouka Suigetsu's Complete Hypnosis 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-sama can quickly create simple or complex illusions to deceive opponents. By creating a decoy of himself, or other illusions to distract his opponents, Aizen-sama 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.
While the illusions Kyouka Suigetsu creates are quite powerful, those with considerable power and skill themselves may be able to note slight differences in the illusions compared to the real thing, even if they are not able to determine what, precisely, is different. Captain Retsu Unohana was able to detect something was amiss with Aizen-sama's fake corpse, even though she did not realize what until seeing Aizen-sama alive. This is how Aizen has deceived many people and killed them, including one of his own allies, the Tres Espada, Tia Halibel.
* Bankai: Not Yet Revealed.