Sousake Aizen never had some form of a backstory, however he was an alone individual (who just so happens to have a really op Zanpakto) then found out about Soul King and the secrets of the Soul Society, só he prepared himself to become the new ruler... and oh boy...

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Aizen was really prepared, he is strong, he is mentally prepared, he is intelligent and has more. He knows all forms of Soul Reaper combat:

Kido: There are separate classes of Kido, Hadō are offensive spells, Bakudo are defensive spells, which also include not only shields, but also communication, maps and more, there are also healing Kido, forbidden Kido which bend space and time (two include instant teleportation and time stop), and unspecified Kido, such as sealing. All of them can also be more powerful with incantation:

Starting with Bakudo:

Bakudo 1 Sai: Basic restrain ability.

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Bakudo 4 Hainawa: The user makes a rope that can restrain their opponent.

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Bakudo 8 Seki: The user can make an energy ball, then when something comes in contact, the energy ball repels it.

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Bakudo 21 Sekienton: It makes a smoke screen so the user can run away.

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Bakudo 26 Kyokko: The user can bend light then makes themselves invisible, which hides their physical form and Reiatsu.

https://bleach.fandom.com/wiki/Kyokkō

Bakudo 30 Shitotsu Sansen: It makes 3 pillars of light that can pin the opponent completely.

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Bakudo 37 Tsuriboshi: It can make a spider web of sorts that connects to any object, like buildings, and has this trampoline sort of effect to it, mainly used to catch falling objects.

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Bakudo 39 Enkōsen: Just makes a circular shield. The only time it was used, it broke:

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Bakudo 58 Kakushitsuijaka: It's one of the communication and tracking Kido, it most likely detects the Reiatsu signature:

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Bakudo 61 Rikujokoro: It's much like Shitotsu Sansen, except it's 6 pillars of light that actually lock the opponent in place, Byakuya used it against a mind controlled Rukia by Zommari, and Zommari was like "I command Rukia to slit her throat" yet even though the rods of light were on her hips, she didn't even move her arms:

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Bakudo 62 Hyapporankan: The user throws one pillar or rod of sorts that then multiplies to several different pillars, also used to pin the opponent in place and immobilizes them as well most likely:

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Bakudo 63 Sajo Sabaraku: A chain of light comes in from the air and can tie the opponent, much like Hainawa except much more powerful:

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Bakudo 73 Tozansho: It makes an upside down pyramid around the user, can be used for cover or shielding:

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Bakudo 75 Gochutetekkan: 5 iron pillars come down from the sky and just smash the opponent into place:

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Bakudo 77 Tenteikura: Used for communication, the user can choose who gets the message and who doesn't, also depends with who the user met because whoever the user met, they're the ones who gets the message:

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Bakudo 79 Kuyō Shibari: Makes 8 black holes around the opponent and another on the chest, which imobilizes the opponent:

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Bakudo 81 Danku: Its a shield (that is as tall as like a building) that blocks any Hadō spell that is level 89 or lower. Aizen used it when he was a lieutenant, he blocked a level 88 Hadō from a captain:

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Bakudo 99 Kin: It makes a.. something that ties the opponent then several pillars come raining down to pin the opponent to the ground, whilst looks basic is one of the highest level Bakudos for a reason:

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Bakudo 99 Bankin: first the opponent gets wrapped around then arrows are shot at the opponent, finally A GIANT STONE ROCK FALLS FROM THE SKY TO CRUSH THE OPPONENT!

https://bleach.fandom.com/wiki/Bankin

We have now several Kido that unnumbered:

Hakufuku: It's a hypnosis, where the opponent sees pedals then falls asleep, it also blocks their spirit energy, because Gin used it on Rangiku and Aizen couldn't sense her spirit energy.

https://bleach.fandom.com/wiki/Hakufuku

Inemuri: It makes the opponent want to fall asleep.

https://bleach.fandom.com/wiki/Inemuri

Tanma Otoshi: Unlike Inemuri, this one actually forces the opponent to fall sleep.

https://bleach.fandom.com/wiki/Tanma_Otoshi

Shibereyubi: It nullifies the opponent completely, like it makes their entire body seem asleep.

https://bleach.fandom.com/wiki/Shibireyubi

Now for the Hados:

Hadō 1 Sho: It pushes the opponent, Byakuya used it on Rukia, but if you noticed, Rukia was free from the little amounts of ice that consumed her, so it has to be a bit powerful:

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Hadō 4 Byakurai: It fires a basic lightning bolt of your finger:

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Hadō 11 Tsuzuri Raiden: if the user is in contact with something that can conduct electricity, they can fire electricity:

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Hadō 12 Fushibi: It can make an explosion, if the user make an entire trap with dynamite, without something like a piece of TNT, I guess it just makes a basic ball of explosion:

https://bleach.fandom.com/wiki/Fushibi

Hadō 31 Shakkaho: The user fires a basic fire ball:

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Hadō 32 Okasen: it's a more powerful version of Shakkaho, except it has a wider spread:

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Hadō 33: Sōkatsui: a more powerful version of Okasen:

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Hadō 54 Haien: It looks like it shoots a really damn powerful fire ball:

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But it was also stated to be existence erasure, as confirmed in the data book:

Hadō 57 Daichi Tenyō: Used in the final arc of Bleach, it can basically allow you to control falling objects, like rocks... kinda like an Earth bender:

Hadō 58 Tenran: With the user spinning their Zanpakto in a circular motion, they can basically make a tornado:

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Hadō 63 Raikoho: The user just makes one big ass lightning bolt:

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Hadō 73 Soren Sōkatsui: It just a Sōkatsui with the damage of two Sōkatsuis:

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Hadō 78 Zangerin: Again in the final arc, the user gets a large amount of energy in their Zanpakto then blasts it away as a circular motion:

Hadō 88 Hiryu Gekizoku Shinten Raiho: The user fires one big ass lightning bolt:

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Now, for the level 90s Kido, as Aizen said, when you try to use a level 90 Kido, they are hard to control even by him, most also requires a sacrifice of sorts:

Hadō 90 Kurohitsugi: Aizen's trademark Kido, it makes a giant box that can stab the opponent in multiple directions, when Aizen used it in his 4th fusion and did the incantation, he stated that it can warp space and time, like black hole:

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And in the final arc, he actually did the Kido again, except even without incantation and being weakened, its actually stronger than the one he used on Ichigo:

Hadō 96 Itto Kaso: One of the 90s Kido that needs a sacrifice, which is the arm of the user, but thankfully Aizen doesn't need to worry about which I'll tell later, anyway, it makes a giant sword of sorts come from the ground that is bigger than the buildings THEN IT EXPLODES! Never got to see it fully Italians because Yamamoto used it on Aizen but Aizen took it like it's nothing:

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Hadō 99 Goryūtenmetsu: The most powerful Kido in the Hadō ranges, to the point Kisuke said that it's forbidden because of how much Reishi it absorbs! It makes 5 dragons appear then... no idea, although, using Brave Souls as a reference, one of the dragons does fire:

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We also have unspecified types of Kido, like El Escudo, which Aizen uses to make a shield appear, there is also Kido that seals, Kisuke used it on Aizen, and several unnamed barriers in the final arc, there is also teleportation Kido and Hanoi which nullifies an opponent's attack.

Aizen also knows Shunpo and it's art, which is a giant enhancement to his speed, or augmentation to his speed. Aizen would know the 3rd art form, which is that he moves at a great speed to avoid an attack and replace it with something, like a captain's coat, then Senka which is he moves behind an opponent and directly attacks the weak points of the opponent, and finally speed clones, in which the user moves so fast that you see clones. Aizen also would know Hakuda, unarmed combat, and Zanjutsu the sword combat. As you also know, Zanpaktos attack the opponents Soul, not just their body... and all of Aizen's attacks, like Kido, also affect/attack/destroy the individual's soul. So if you don't have astral defenses, you will be one shot.

Now for the greatest part about Aizen, his Kyōka Suigetsu, his Shikai. It's a hypnosis, but it's the most powerful, after releasing his sword with the enchantment of "Shatter, Kyoka Suigetsu" it controls ALL your senses, your sense of sight, sense of smell, touch and even your sense of time, even your Reiatsu sensing, you can't even detect Aizen!

You just need to look at Aizen's release once and you're under the influence of Kyōka Suigetsu. In the final arc, Aizen has evolved to where he fused with his Zanpakto, and he even managed to hypnotize Yhwach, he didn't even unsheathe his sword or anything, yet Yhwach was like "wait, is my sense of time disturbed?" and Aizen even used it to disturb Yhwach's sense with the Almighty, like it caused Yhwach to pick the wrong futures! So you just look at Aizen and you're under his Hypnosis! Aizen even is smart enough to let his opponents win on purpose, so that they won't be suspicious of anything!

Aizen has never shown his Bankai... but why does he need to?

Aizen is also really damn smart, like the second smartest. He literally found out the Hogyoku's true power from just ONE piece of data!

He even planned out the WHOLE series of Bleach from hundreds of years ago!

Speaking of the Hogyoku, it has a mind of its own and can bend reality, like it can do anything it wants. Aizen used it to gain evolution, and Hogyoku was able to give Aizen, not just more power to beat the opponent, but also things like immortality, like Aizen can regenerate from being attacked by Gin's Bankai, which made explode from the inside out on a cellular level!

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Being sliced in half by Final Getsuga Tenshou Ichigo, who was literally WAY stronger than Aizen!

The Soul Society couldn't even execute him as they state. If we go with common sense and say the used the Sogyoku, which is a Phoenix Blade that has the power of one million Zanpaktos and can destroy the Soul Society when fully powered up, it definitely didn't work.

The Hogyoku also gave Aizen teleportation!

All that the Hogyoku would do, is just read Aizen's deep desires and it will loyally give it to Aizen, even if they are separated, the Hogyoku will read Aizen's desires.

In fact, even when weakened, Aizen was still able to fight Yhwach:

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Meaning, he would scale to Yhwach, who can merge all the realms in Bleach at the same time! As everyone knows, the realms have multiple stars and multiple galaxies, and they were one universe before being split, so this is a universal+ feat, because Yhwach also needs to manipulate the time and space, the reason being because Shinigami and Hollows always need speaking to just go from one realm to another, they never like fly outside or anything. You can get it to low multiversal, if you use Memories of Nobody movie and Hellverse, in which The Valley of Screams and Hell are implied to contain infinite souls in them. As for speed, Aizen can scale to Yhwach and Ichigo, both have impressive MFTL feats, like Ichigo blitzing the Quincy girls who dodged the Aushraleon light, which crossed the distance between the Soul Society and the Soul Palace in two seconds, a distance that took Ichigo 9 hours to complete, and he outpaced Balas in a weaker state, which are 20 times faster than Ceros, and it was also said that with Shunpo, it would take 1 week to complete the distance, according to one of the members of the 0 squad, who by lore are stronger than all the Gotei 13 COMBINED!

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Aizen even planned to shoot down the Soul Palace!

Aizen would even be able to resist hax from weaker opponent:

And Aizen can even tell he's under illusions, because Shinji used his Shikai on Aizen, the Shikai is where it reverses your senses! So up is down, left is right! Aizen was able to tell that he was under an illusion, like he just said "Oh, you used a hypnosis on me? How fodder"

He even had Shinji under his hypnosis the entire time!

Aizen also evolved to a point where he can obliterate weaker beings BY JUST STANDING THERE! Like, he was weakened yet he almost incinerated a Shinigami who was going to put in another seal!

Aizen can not just evolve and get more powerful with the Hogyoku, but he also get more powerful during fights:

Well, that's it for today guys.

MarioandSonicfanboy here are some Aizen analysis for you.