Kyoka Suigetsu (鏡花水月 lit. Mirror Flower Water Moon) is the Zampaktou used by Sosuke Aizen.
The Shikai of Kyoka Suigetsu has a release command of "Shatter" used for both activating and deactivating it, as Aizen revealed what seemed like his body to be his sword.
The only special ability revealed is Kanzen Saimin (完全催眠, Complete Hypnosis). It controls the senses so that the victim misinterprets the form, shape, size, texture and smell of any object.
Upon seeing the Shikai of the Zampaktou once, the victim is put under its hypnotic power, and subsequent activation of the ability will still be effective.
Aizen need not show the action of releasing the Zampaktou, only its released state.
His lesser abilities include creating an imaginary space he can hide inside. When Aizen used it when he was still a vice-captain, he was easily discovered by Shinji via unknown means.
Aizen has also shown to be able to use throw the target's sense of time. However, this was after he unlocked the true power of his Zampaktou, so it was unclear whether this is part of Kanzen Saimin or a separate ability on its own.
The Bankai of Kyoka Suigetsu was never shown. However according to Yoruichi, every captain, which includes Aizen at that time, has mastered both Shikai and Bankai.
Kanzen Saimin, once activated, stays activated as long as the target and victim is in range. By carrying Aizen's Zampaktou, a Shinigami is able to walk around visually disguised as Aizen, even when Aizen himself is not around.
Aizen's large amount of Reiyoku allows him to create near flawless illusions. Even with knowledge about the ability, it is almost impossible to evade in a combat setting even for captain-level fighters.
Kanzen Saimin is also an ability that has an area of effect. All of the victims who has seen the Shikai release will be affected by Kanzen Saimin, and even Aizen as the user himself has no say in this.
With the final evolution of Hogyoku before Aizen was sealed, Kyoka Suigetsu was fused with Aizen himself. Aizen no longer need to sheath his sword for his release, and the mere sight of him allows Aizen to put his target into Kanzen Saimin.
With that, Aizen no longer controls what the victim sees. He was able to cast illusions about the new Bankai of Renji and Ichigo, despite never seeing them before. This might be due to Hogyoku's ability to realise desires, which projects the illusion of what Yhwach imagines rather than Aizen.
When Aizen was still a vice-captain, he already had massive Reiryoku, enough to fool Shinji as a captain. However, Aizen stated that even the true power of his Zampaktou, Kanzen Saimin, can be seen through when given enough attention. This can be done by observing strange behavior from disguising as someone else.
Those with considerable power and skill themselves may be able to notice slight difference in the Reiatsu. Unohana was suspicious of the illusion of Aizen's death. However, it might also be possible due to sheer (woman's) intuition, as neither of them implied that she spotted any flaw.
The more famous weakness is touching the blade before Kanzen Saimin is activated. According to Gin, that is the only way to escape it completely. This method does not work during when Kanzen Saimin is activated, as seen from the Unohana example.
Yamamoto was able to determine the real Aizen by sensing the Reiatsu of the Zampaktou that Aizen used to stab him. However, this is likely a consequence of touching the blade, which allowed Yamamoto to escape its power.
Aizen is also unable to control who would be affected by Kanzen Saimin. The ability indiscriminately puts all whom has seen the release under the illusion.
After absorbing the Hogyoku, Aizen no longer decides what the illusions entails, which is now seems to be completely up to the user. While this might remove the human error, it also decrease the flexibility in its usage.
The illusion on Yhwach ended when Yhwach penetrated Aizen's heart. This can be interpreted a few ways on why the illusion stopped.
- Since Aizen is fused with Kyoka Suigetsu, touching his body or the Hogyoku is a condition to stop the illusion. This is quite unlikely since as said above, touching the blade while Kanzen Saimin is active will not work.
- Causing enough damage to the user. Or a lack of power.
In Can't Fear Your Own World III, it is said that those with higher Reiryoku must be put under the spell again once Kanzen Saimin is being deactivated.