Name : Sōsuke Aizen
Gender : Male
Prior to his departure from Soul Society, he 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. However, 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. As he had previously revealed to Renji Abarai, the Aizen they all knew never existed and this would seem to be a visual representation of that claim.In Hueco Mundo, Aizen is first seen wearing Arrancar clothes over his Shihakushō. Afterwards, after Orihime Inoue is taken to Las Noches, Aizen wore full clothing similar in style to the Arrancar, complete with a pink sash around the waist.
Aizen's appearance changes again upon his initial defeat by Ichigo Kurosaki and being sealed. Aizen loses all of the physical changes made by his transformation and returns to the physical form he had prior to transforming.
Personality : Despite his current villainous status and former position of authority, Aizen 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 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 (both past and present), and is willing to use almost everyone around him as pawns to achieve his means. In his own words, Aizen claims to be unaffected by any form of morals or ethics, as he views them as mere restrictions on his potential. He is, therefore, willing to stop at nothing to further his selfish ambitions, including manipulating or killing anyone if he deems it to be necessary. He is fond of long, complicated, and somewhat convoluted plots, and frequently appears to toy with his underlings for amusement. Unlike most of the other Shinigami who appear to have some kind of honor and kindness, including some Espada, Aizen lacks genuine compassion and honor.
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's sudden departure.
As the commander of his army of Arrancar, Aizen controls his 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 there to further Aizen's ambitions. For more rowdy individuals like Grimmjow Jaegerjaquez, he keeps them in line through intimidation and mind games. The Arrancar Aisslinger Wernarr also claims that they follow him because he has no fear, something which beings born from fear and despair find inspiring. Other Arrancar occasionally follow him for reasons unique to themselves, such as Szayelaporro Granz, who follows him 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's manipulation. Ichimaru has also shown that he has some sense of Aizen's emotions and feelings, correctly identifying that Aizen was enjoying the invasion of Ichigo and his friends and their successes within Las Noches, as well as picking-up on the fact that Aizen had registered the arrival of Hinamori in battle of Fake Karakura Town and was surprised by it. By contrast, Kaname Tōsen has shown no such understanding and has fallen victim to one of Aizen's more minor mind games when Aizen off-handedly 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 the Soul Society. As he and his fellow traitor captains were departing from the Soul Society, he told Jūshirō Ukitake that "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". To do this, he needs the Ōken (王鍵, King's Key), 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 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.
According to Kisuke Urahara, since mastering the Hōgyoku, Aizen has become less cautious in battle, noting that before he subdued the device, Aizen would not have allowed him to use level 90 Kidō, Aizen admits that his new powers allow him to let his guard down. Also due to this new-found power, he has developed more of a seemingly 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 also 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 that he doesn't understand; for example, when witnessing Ichigo in his "Final" form, Aizen becomes furious because he is unable to comprehend how a Human has surpassed him.
History : At an unknown point in the past, Aizen became part of the 5th Division under Captain Shinji Hirako. He invested himself in the research and creation of the Hōgyoku even before Kisuke Urahara. To this end, he sacrificed hundreds of Shinigami and hundreds of Rukongai citizens who bore Shinigami talents to his own Hōgyoku. But it was not satisfied with this. He is seen standing in a clearing with three other Shinigami who are kneeling before him by Gin Ichimaru. One of the Shinigami hands Aizen the Hōgyoku. Aizen puts the object in a glass and then hands it back to the Shinigami. Aizen and his subordinates leave an unconscious Rangiku Matsumoto a short distance away. Aizen eventually caught his captain's attention, and was promoted to a lieutenant in order for Shinji to watch over him closely. However, Shinji's action only helped further the realization of his plans.
Aizen with Shinji Hirako, his 5th Division captain.
110 years ago, Aizen was seen making his way to the 5th Division living quarters of his captain. He and Shinji had a discussion about style and trends before they made their way to the promotion ceremony being held at the 1st Division headquarters. As they got there, he witnessed Hiyori Sarugaki attack his captain and the two got into a childish argument. He followed behind while the other captains congregated and had a brief discussion on the status of the Gotei 13. Overhearing Shunsui Kyōraku mention that the previous captain of the 12th Division was promoted, Aizen interrupted and asked what Kyōraku meant by it. Aizen initially conjectured that the promotion was to Central 46. However, Shunsui explained that it had nothing to do with the Central 46 and that the previous Division Captain Kirio Hikifune had been accepted into the Zero Division also known as the Royal Guard. Aizen was left surprised at the revelation.
Shinji reveals Aizen's presence.
The following night, after Kisuke Urahara's appointment as the new captain of the 12th Division, Shinji had a short talk with him about his new role as captain. While leaving, Shinji asked, "How long do you plan on watching us from over there?" Using his hand, Shinji revealed Aizen concealed behind some form of camouflage. Aizen complimented his captain and asked him when he sensed him there, to which Shinji replied ever since he was in his mother's womb, and ordered Aizen to follow. Aizen noted that won't do at all. Aizen then commented on how scary a person Shinji was, to which Shinji replied that Aizen was the scary one.
The very next night, Aizen saw Gin Ichimaru killing the 3rd seat of their division, and informed him that he was impressed by his skill. Aizen then asked him how he found the fight, to which Gin stated he found it pathetic.
9 years later, Shinji and Aizen were walking around the Seireitei when they came across Urahara, Hiyori, and 3rd Seat Mayuri Kurotsuchi. While Shinji once again engaged in a childish fight and argument with Hiyori, Aizen asked Urahara if he had heard the news to which Urahara told him he hadn't. Shinji proceeded to take up the tale and tell Urahara about the series of strange deaths that had taken place in Rukongai. Shinji also informed him that the 9th Division had been sent to handle the investigation.
Kyōraku spots "Aizen".
With the disappearance of the 9th Division, Captain-Commander Yamamoto called an emergency meeting and assembled a team to investigate. After coming into contact with Hollowfied 9th Division members the investigation team was dispatched by an unseen assailant. Aizen was seen wandering around at night by Captain Shunsui Kyōraku and some other Shinigami on guard, while the investigation team was still out searching.
It was later revealed that the 5th seat of the 9th Division, Kaname Tōsen, was behind the betrayal of the 9th Division, having stabbed Kensei Muguruma in the back and killed Heizō Kasaki, Shinobu Eishima, and Tōdō Izaemon, who had accompanied Kensei. He subsequently subdued the team sent to locate the missing 9th Division members using his Bankai. When Captain Shinji Hirako of the 5th Division, asked him why he betrayed his captain, he was surprised to be answered by his own lieutenant, Aizen, who stated that Tōsen didn't betray anyone and that he is, in fact, very loyal, and he just faithfully followed his orders.
Aizen reveals his betrayal to Shinji.
Shinji admits he was not as surprised as he seemed to be, and Aizen asked if he suspected him, stating that he had expected that much from his captain. Shinji then explains how he had known something was dangerous about Aizen and that he couldn't be trusted. It was for those reasons that Shinji chose him to be his lieutenant, so that he could keep an eye on him. Aizen told him that he was grateful, as due to his deep seated doubts he didn't suspect a thing. When Shinji states that he already made it known that he had suspected him, Aizen elaborated stating that he didn't realize that for the whole past month, it wasn't him walking behind him. Aizen proceeded to explain the power of his Zanpakutō, Kyōka Suigetsu. Aizen informs Shinji that if he was as close to Aizen as other captains were to their lieutenants he would have been able to see through it by picking up on subtle differences between Aizen himself and Aizen's double. It was his lack of trust that kept them from getting close and as such, allowed Aizen to fool Shinji. He also explains that he could have always refused to be Shinji's lieutenant, but decided against that action because his captain's skepticism made the position ideal to carry out his plans. Aizen thus laid the blame for Shinji and his comrades' current predicament at the feet of Shinji. Provoked by this, Shinji's anger triggered a speed-up in the Hollowfication process. Aizen thanked him for being so easily provoked and made a note of the observed connection between Hollowfication and agitated emotions.
Aizen confronted by Kisuke
Aizen is confronted by Kisuke and Tessai.
As he got ready to kill Shinji and the others, Kisuke Urahara and Tessai Tsukabishi arrived to stop Aizen from dispatching his captain. Aizen and Urahara had a brief discussion about what was transpiring, with Aizen acting innocent at first until Urahara notes that the investigation team doesn't have simple injuries, but were in fact going through Hollowfication. Aizen commended Urahara on his deduction and revealed himself to be behind the disappearances in the Rukongai. As Aizen had been engaging in early experiments on Shinigami/Hollow hybrids, which involved using an unknown device to cause spirits to transform into Hollows, he was responsible for turning his former captain and seven other Shinigami into Visored, though Aizen refused to go into any of those details.
Aizen proceeded to leave the scene of the crime with his henchmen, stating that "there's nothing more to do there". To stop them from retreating, Tessai performed the Hadō #88, Hiryū Gekizoku Shinten Raihō spell against them, only to be countered by Aizen using Bakudō #81. "Dankū", allowing them to escape.
Captain Aizen and Lieutenant Ichimaru.
Some point after the Hollowfication incident, Aizen became Captain of the 5th Division, and eventually revealed to Gin the secret to avoiding Kyōka Suigetsu's complete hypnosis. He later made Gin his lieutenant. Aizen continued to experiment on Hollows to transform them into Shinigami and he created Hollows that were close to Shinigami, such as Metastacia and Menos Hollows capable of concealing their Reiatsu. While testing out the mentioned breed of Menos, he and Gin saved the then first year academy students Momo Hinamori, Izuru Kira, and Renji Abarai from them. Aizen's interest was then piqued by them. Realizing their potential, Aizen took them all into his division. While the obedient and loyal Kira and Hinamori became the lieutenants of the 3rd and 5th Divisions under Gin and Aizen, respectively, Renji proved to be too rebellious and was transferred to the 11th Division (later transferring to the 6th Division to become that squad's lieutenant).
Gin 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 always thought of Gin as his subordinate.
Aizen greets Baraggan in Hueco Mundo
Aizen greets Baraggan in Hueco Mundo.
At some point during their captaincy, Aizen, Gin, and Tōsen, secretly visited Baraggan Louisenbairn, King of Hueco Mundo and Lord of Las Noches. He and Baraggan engaged in a light conversation, with the 5th Division captain asking Baraggan to look at his Zanpakutō. He then continued by asking Baraggan if he was happy in his position and if he would join him in a new world where he will be granted more power. Baraggan dismissed this notion, stating that there was no power higher than his. Aizen then released his Shikai and showed Baraggan the decimation of his army, thus subjugating him in turn. This created great enmity between Aizen and Baraggan, who promised to seek the Shinigami's destruction for all eternity.
Sometime after the Hollowfication Incident and during his and Isshin Shiba's time as captains, he, Gin, and Kaname were still working on their Hollowfication experiments, as well as attempting to locate Shinji Hirako's group. He told his co-conspirators to remain patient, reassuring them that everything will go as planned. 
Soon after this, Aizen and his two comrades watched the fight between Isshin and one of their Hollow creations on a monitor, with Aizen wondering why Isshin was present there. Tōsen states that he did not report that he was going, with Gin commenting that this could cause them immense trouble. Intrigued by this interesting turn of events, Aizen referred to Isshin's arrival as an unexpected harvest and asked if it was possible for him to watch the battle at a closer distance. Later, when he arrives at the scene of the battle with Gin and Tōsen, the three of them each wearing a Reiatsu-concealing cloak, Aizen cuts down Isshin from behind. After commenting on the quality of Urahara's cloak, as well as how simply kneading Kidō into it will let the wearer blend into the air, Aizen prepares to leave, stating that Isshin won't be able to use Bankai in his current state and that the Hollow Experiment was a failure if a captain could take it out using just Shikai. After Tōsen expresses his desire to call the sample "White", as this was their first experiment created based on a Shinigami's soul, Aizen comments that it's ironic calling the Hollow "White" because of its inner whiteness while its outside is completely encased in black. After observing the fight for a time, Aizen and his cohorts notice the involvement of a Quincy in the fight, which surprises the three of them. After Tōsen decides to eliminate her, Aizen quickly stops him from killing her, eagerly stating that they should see what happens next.
Following White's defeat and destruction, Tōsen expresses his shock and frustration at the events, calling it a failure. Aizen calms him down, saying that the results were interesting and also expressing curiosity on how events will further unfold.
Aizen became aware of the existence of Ichigo Kurosaki when he was born. Understanding the significance of his birth, he had decided to aid his development for his plans. To this end, he had Rukia Kuchiki stationed in Karakura Town. He also put into place obstacles that would force Ichigo to grow stronger, including the Hollow that attacked his family and the Menos Grande that appeared when Uryū Ishida used the Hollow bait
Clan : Espada
Ability:Genius Intellect: Arguably, the most fearsome trait of Sōsuke Aizen is his tremendous intellect. He is well-versed in the history, strategy, and tactics of Soul Society, and is privy to knowledge unknown to many, such as the existence of the Ōken, as well as the obscure method in which to create one. He has also created several Hollows and Arrancar while avoiding arousing any suspicion from the majority of his fellow Shinigami. For over 110 years, he had been engaging in experiments that involved unique and highly dangerous manipulations of spirit energy and spirit particles, including experiments involving the Hollowfication of Shinigami, something that had never been done before. He has an innate knowledge of the Hōgyoku, rivaling that of its creator, Kisuke Urahara; he even knows how to destroy it. He is able to figure out Shinji Hirako's Shikai ability's true nature and deduce that it produces an optical illusion, allowing him to counter its effect. He effectively deduces the true nature of the Hōgyoku via trial and error of situations that revolve around it.
Master Manipulator: He has shown himself to be a very crafty and cunning man since his public betrayal of Soul Society. He is able to deceive and manipulate others around him in different ways for a variety of purposes. For years, he was able to convince everyone around him that he was a kind-hearted man with the best intentions for Soul Society, while 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 master 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 which involved moving people around like pieces on a chessboard and 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, allowing him to use her abilities to extend the shelf life of the Hōgyoku, and, after the departure of Gin, Tōsen and himself, trapping the Ryoka and four captains in Hueco Mundo, making it easier for him to accomplish the goal of his first plan; creating the Ōken and gaining access to the Royal Dimension.
Master Tactician: He has also shown himself to be a masterful and flexible tactician, capable of adjusting seamlessly to any given combat situation, if not planning out events well in advance. He understands the strengths, weaknesses and mannerisms of his enemies, and creates strategies to achieve victory; this includes modifying an Arrancar for the sole purpose of defeating Yamamoto, a Shinigami whose abilities surpass his own. Though it can be said that the majority of his tactics rely heavily on his Zanpakutō's special ability, there are few tactics better 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 highly cautious in battle, placing special protections on himself so as to leave no blind spots.
Aizen using Hadō #90: Kurohitsugi (Black Coffin).
Kidō Master: Aizen has great proficiency in 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 fellow Captain Sajin Komamura. He has even used Dankū (ranked 81) to block another Kidō attack by Tessai Tsukabishi, who was captain of the Kidō Corps and logically the most proficient Kidō master in the Soul Society at the time, while Aizen himself was only a lieutenant. He can produce powerful protection spells while in battle.
Immense Strength: He is powerful enough to stop Ichigo's Bankai with just one index finger (albeit Ichigo was already battle-worn from fighting Byakuya Kuchiki) and almost cut him in half with a single swing of his sword. He stopped Renji's Shikai with his bare hand and broke it with one swing, injuring Renji in the process. He was also able to stop Komamura's Shikai with his bare hand, proving his strength was more than enough to combat someone of the same level. Aizen further displays his strength by effortlessly cutting through the sword and wrist of Komamura's Bankai.
Immense Spiritual Power: Even by captain standards, Aizen possesses tremendous spiritual power. Marechiyo Ōmaeda commented that Aizen's very presence is monstrously overwhelming. A hint of his level of spirit power is given when he turns Wonderweiss Margela into an Arrancar before activating the Hōgyoku; he states that despite its half-awakened state, it can be fully activated when temporarily fusing with someone with twice as much spiritual power as an average captain-level Shinigami (referring to himself). The force of Aizen's spiritual power is also great enough to, without any visible effort, bring the 6th Espada 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's Reiatsu is powerful enough to nullify abilities, such as when he negated Suì-Fēng's Nigeki Kessatsu technique. His Reiatsu is violet.
Master Swordsman: While the full extent of his prowess is unknown, Aizen can effortlessly fight back against an opponent's Shikai and Bankai with his own blade still sealed. He can perform tremendously powerful and precise strikes, capable of cutting through very sturdy targets, such as Renji's Shikai. He easily cut down four captains (two which used their Bankai) and four Visored (with three being former captains of Gotei 13) with ease.
Shunpo Master: He is perceived to be so fast that one cannot discern even being attacked by him until after it has taken place. Aizen is shown to easily outmaneuver Ichigo's Bankai and Hollow mask-enhanced speed, to the point where 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 evading 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.
Hakuda Master: While preferring to finish his fights quickly with his Zanpakutō or Kidō, Aizen is a highly capable unarmed fighter. Repeatedly, Aizen has effortlessly caught his opponent's weapons with a single bare-hand. While fighting offensively, he has shown tremendous agility and dexterity, able to combine both sword and barehanded skills to simultaneously strike with his sword and kick opponents with great force.
Immortality: After being hit by Ichigo's Mugetsu, he is shown to have somewhat regenerated from the damage, though most of his new form's features have been destroyed and do not return. He is later confirmed by the Central 46 to having been left immortal by his interaction with the Hōgyoku.