Mysteries surrounding Aizen's Bankai crown him as the greatest villain.
The reason AIZEN never uses his BANKAI – ODBS
Sosuke Aizen from Bleach stands out as one of the greatest villains of all time in the anime world, probably because of his undiscovered Bankai. Despite being so powerful, Aizen's Bankai remained a secret. His Bankai is one of the mysteries of Bleach, resulting in fans often coming up with new theories about his Bankai and its likelihood. Some of them believe that this is why Aizen became the greatest villain of all time.
By keeping his powers a secret, Aizen ensures that no one can learn what his true abilities are. He is never shown to reveal Bankai, this makes his opponents always wonder how far he can go. This is one of the main reasons that make him a nightmare for his enemies.
Aizen's Zanpakutō, Kyōka Suigetsu is already a terrifying weapon. Aizen can hypnotize his opponents while manipulating his opponents five senses. This allows him to create perfect illusions, Kyōka Suigetsu's immense capacity already adds to him and establishes him as a formidable foe. Considering the power of Kyōka Suigetsu, his Bankai has great potential. There could also be another explanation for his Zanpakutō not allowing him to use his Bankai.
