Sōsuke Aizen (藍染 惣右介, Aizen Sōsuke) is the former captain of the 5th Division in the Gotei 13. He later leaves the Soul Society with his followers, Gin Ichimaru and Kaname Tōsen. His lieutenant was Momo Hinamori. After waging war against Soul Society with an army of Arrancar, Aizen was defeated by Ichigo Kurosaki and then sealed away by Kisuke Urahara, before later being imprisoned for his crimes.

Hōgyoku (崩玉, Crumbling Orb, Viz: "Breakdown Sphere""): Aizen has stated that he arrived at the conclusion of the Hōgyoku before Kisuke Urahara, however, it ended in failure. In order to finalize the Hōgyoku, the souls of hundreds upon hundreds of Shinigami and hundreds and hundreds of Rukongai citizens who bore Shinigami potential needed to be sacrificed to it, but it still wasn't satisfied. According to the experimental data, Kisuke Urahara's Hōgyoku was also incomplete. Therefore, Aizen decided to steal that Hōgyoku and present it to his own.Produced as a result of Aizen's Hollowfication research, this Hōgyoku is what gave the Visored their Hollow powers. This process seems to spread as some form of infection through wounds, which only has the desired effects on souls with high-level spiritual energy and not souls with low-level spiritual energy. Those infected experience great sickness and fatigue, until they vomit up a white liquid that forms the beginnings of their Hollow mask, which then proceeds to transform their bodies and warp their minds. Other than that, all that is known is that the process is accelerated by aggressive emotions. The conclusive result turns a Shinigami into a primarily Hollow-like humanoid, which is a highly unstable transformation causing mindlessness and aggression.
Caja Negación (反膜の匪 (カハ・ネガシオン), kaha negashion; Spanish for "Negation Box," Japanese for "Anti-Membrane of Negation"): This cube, when used on a subject, traps them in an alternate dimension from which there is no escape, however, this assumes it's being used as intended. More powerful Arrancar, namely the Espada themselves, can escape, given enough time.
Hollow Research and Development: During his tenure as a Shinigami and after his defection, Aizen spent much time researching Shinigamis and Hollows in order to break down the boundaries between them. Because of this, he was able to create Hollows with Shinigami-like abilities. He created Hollows with the ability to hide their reiatsu like the ones who attacked Renji Abarai, Izuru Kira, and Momo Hinamori during their training, Hollows, like Metastacia, with the ability to shatter a Shinigami's Zanpakutō and fuse with them, and even a Hollow using a Shinigami's soul as a base. However, according to Aizen, these all ended in failures. After his defection, he began using the Hogyoku to create Arrancars with far superior ability than normally occurring Arrancars. Aizen was also able to give Hollow powers to a Shinigami, namely Kaname Tōsen who gained both a Hollow's mask and a Resurrección.

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.
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.
Zanpakutō:None. Aizen's Zanpakutō eventually disintegrates, as he claims that the Hōgyoku has determined he no longer needs it.

(To Tōshirō Hitsugaya) "You should learn this well, Hitsugaya-kun. Admiration is the emotion furthest from understanding."

(To Renji Abarai) "The Sōsuke Aizen you knew...never existed to begin with."

(To Jūshirō Ukitake) "No one stands on the top of the world. Not you, not me, not even gods. But the unbearable vacancy of the throne in the sky is over. From now on...I will be sitting on it."

(To Shinji Hirako) "The betrayal you can see is trivial. What is truly fearsome, is the betrayal that you don't see, Captain Hirako."

(To Tier Harribel indirectly) "I never would've imagined that the power of the Espada I so painstakingly assembled would be less than mine alone."

(To Shinji Hirako) "Trusting in someone is the same as depending on them. That is something the weak do. We have no use for it."

(To Shinji Hirako) "All living creatures place their faith in someone more powerful than them and they cannot survive unless they blindly follow that person. The recipient of that faith then seeks out someone in an even higher position in order to escape from the pressure. That person then seeks out someone even more powerful that he must put his faith in. In this way all kings are born and in this way all Gods are born. Do not believe in me yet, Hirako Shinji for now I shall slowly teach you who the God is that you will put your faith in. Believing in me comes after that."

(To Ichigo Kurosaki) "Why are you putting so much distance between us? If you want to make sure it hits me, then you should get close and fire. Or is it that you're afraid of letting even a part of me out of your field of vision by getting close? If that's the case, then it's a foolish thought. Distance only has meaning in a fight between equals. With you and I, distance holds no meaning at all. Watch... If I do this, my hand is almost instantly at your heart."

(To Sajin Komamura) "You thought you could destroy me by striking with power? How naive. ...No, perhaps we differ in our basic understandings of the word "power". I shall enlighten you. This is what is known as "power"."
"I will not take your lives. With the measure of power that you possess you are unlikely even to lose consciousness from those wounds. Stay there, helpless and defeated and observe carefully as this battle reaches its conclusion."

(To Genryūsai Shigekuni Yamamoto) "Your Ryūjin Jakka is indeed the most powerful Zanpakutō in existence. There is no doubt about that. Were I to fight against it directly, it would likely overpower even me. However, if one abandons all other abilities and focuses his strength entirely on a single specialized skill, then he can overcome even the greatest of powers."

(To Genryūsai Shigekuni Yamamoto) "Cruel? A soul that's become a Hollow has no purpose. It does nothing more than meaninglessly hunt down souls to prey on. What is so cruel about granting meaning to such an existence? I would say that you're the one who's cruel for smashing that soul to pieces."

(To Genryūsai Shigekuni Yamamoto) "To you, I will not say "I will not kill you". You who are the very history of Soul Society itself and as such, you alone will die by my sword."

(To Ichigo Kurosaki) "I suppose it is only to be expected. You have my pity. There is no such thing as 'truth' or 'lies' in this world; there never has been. There is only plain, hard facts. And yet, all beings who exist in this world take only those 'facts' that are convenient to them, and take them to be the 'truth'. They do so because they know no other way to live. However, for those powerless beings that make up the majority of this world it is those 'facts' that are inconvenient for their own self-affirmation that make up the real 'truth'."

(To Isshin Kurosaki) "...But living creatures are strange. They are made in such a way that they can actualize only what their minuscule minds wish for."

(To Kisuke Urahara) "Allow me to be careless, as I don't have the need not to be. I can feel the strength of the Hōgyoku in my body. My strength exceeds my older self. I'm now invincible so I don't have the need to dodge a level 90 Kidō anymore."

(To Gin Ichimaru) "Reason exists for those who cannot go on living without clinging to it. Now let's go... to the edge of reason."

(To Gin Ichimaru) "Fear is necessary for evolution. The fear that one could be destroyed at any moment. Thank you, Gin. Thanks to your efforts, I have finally risen to an existence that surpasses both Shinigami and Hollow."

(To Kisuke Urahara) "That is the theory of the vanquished! Victors must always speak of the way the world should be, not the way it is!"

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