Bleach was one of the "big three" of its era, and will return in October 2022 with the anime's "Thousand-Year Blood War" story arc. Newcomers and veteran Bleach fans alike know the title features some of anime's best villains -- most of all, the scheming traitor Captain Aizen, about whom there are some intriguing theories.
Bleach fans on Quora developed a theory that Captain Aizen didn't merely keep his zanpakuto's bankai form hidden -- he never had one at all. Most Captains need bankai to keep their rank, but the crafty Aizen Sosuke can bend the rules if it's for the benefit of his master plan -- and no one was ever the wiser.
Bleach Theory: Aizen Used An Illusory Bankai
Among the Soul Reapers, there are strict requirements for obtaining the rank of Captain. Only a few methods exist to obtain the rank, such as defeating a current Captain in a battle, which is what Captain Zaraki Kenpachi did. Another route is for a candidate to demonstrate their proficiency with bankai, a zanpakuto's ultimate release, among other tests. With Kenpachi as the sole exception, all Soul Reaper Captains as of the "Soul Society" arc were capable of bankai, such as Senbonzakura Kageyoshi and Konjiki Ashisogi Jizo. Captain Aizen certainly made good use of his shikai's hypnotic powers in the "Soul Society" story arc, but he didn't use bankai at all. For that matter, he also failed to use bankai during the Fake Karakura Town battle and the final fight against the Quincy king Yhwach.
According to the fan theory, Aizen doesn't have a bankai at all -- just the shikai, Kyoga Suigetsu. Normally, this would bar him from becoming a Captain, but Kyoga Suigetsu's illusions are so powerful that Aizen could have used the shikai release to hypnotize everyone and fool them into seeing whatever bankai Aizen thought would satisfy the examiners. Every Captain would be present to test Aizen as a Captain candidate, and that would be the perfect opportunity for Aizen to put them all under Kyoga Suigetsu's power and reveal his illusory bankai.
These illusions don't have a time limit, so Aizen could keep that illusion going right up until his betrayal, when it no longer mattered. Another factor in Aizen's favor is that Captains only rarely have occasion to use their bankai in battle, so Aizen could keep his "bankai" sealed away and use his Captain exam as the sole instance of it being used. For the other Captains, that would be sufficient, and no one would question Aizen's qualifications for his rank. He was popular for being a kind, artistically inclined master of kido spells, not a bankai enthusiast.
Why Aizen Would Create An Illusionary Bankai
At first, it may seem odd that the talented and powerful Captain Aizen would need to fake a bankai to become a Captain, since his skills with swordplay, advanced kido spells, the flash step and more are all authentic. Then again, Aizen was always fixated on breaking the natural barriers of strength and obtaining unique power, and if he were naturally born without the potential for bankai, it would drive him to compensate by obtaining greater power, such as the forbidden Hogyoku. In his final fight against Ichigo, Aizen was practically unstoppable as he fused with the Hogyoku, with no need for his zanpakuto at all. This more than made up for a lack of bankai, and no doubt Aizen was planning for that years in advance.
As for the rank of Captain, Aizen needed more than just the Hogyoku to achieve his goals. Before creating his own Hogyoku or stealing Kisuke's, Aizen needed access to advanced research data and lab funding. He probably couldn't do that as a Lieutenant, and he wouldn't ask his then-Captain Hirako Shinji to conduct this research for him. Being a Captain comes with more than just authority -- it also grants the Soul Reaper lofty privileges, and that's what Aizen needed to conduct the historic project of developing and using the Hogyoku. So, if he naturally lacked bankai, he would use his shikai to fake one and gain the rank of Captain, along with its much-needed privileges. Aizen has always been a ruthless and calculating schemer -- he wouldn't hesitate to fool every single Soul Reaper officer to get what he wanted.