Kyoka Suigetsu (鏡花水月 lit. Mirror Flower Water Moon) is the Zanpakuto (斬魄刀 lit. Soul-Cutter Sword) used by Sosuke Aizen.
Like all Zanpakuto, the sword itself shares the same name as its Shikai (始解, Initial Release). Kyoka Suigetsu's Shikai form resembles a regular Zanpakuto except for its hexagonal "tsuba" or guard.
Interestingly, Aizen's Kyoka Suigetsu is always seen in its Shikai form even when not in combat. Although not officially listed like Ichigo's Zangetsu, it is very likely that it is a Constant-Release Type (常時解放型, Jōji Kaihō Gata), as Aizen too has immense amount of Spiritual Energy.
The Shikai of Kyoka Suigetsu has a release command of "Shatter". Like all Zanpakuto, the act of releasing the Zanpakuto reveals its unique Shikai form. Should the Zanpakuto be activated and disguised as something else, the act of releasing it also reveals its true form, effectively deactivating the spell.
The one special ability of its Shikai is Kanzen Saimin (完全催眠, Complete Hypnosis). It is a hypnotic spell that controls the five senses so that the victim misinterprets the form, shape, size, texture and smell of any object.
Any target can be chosen to be disguised into anything or anyone else without fulfilling any condition. However, only those who have seen its Shikai state will be affected by Kanzen Saimin.
It is not clear if the effects of Kanzen Saimin affects the senses of the user himself, though Aizen will know which is an illusion even if he is affected.
Aizen was able to adapt to Shinji's ability rather quickly, which might suggest that Kanzen Saimin affect his own senses as well.
His lesser abilities include creating an imaginary space he can hide inside. Although it was never revealed on how it works specifically, it might be the power Aizen told Isane that uses diffused reflections of fog and water.
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, it is unclear whether this ability 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.
Right before Aizen was defeated, his final evolution was fusion of the Zanpakuto into his body. This is similar to Ichigo's Final Getsuga Tensho form (fan dubbed Mugetsu even though that's the name of the attack) as he became Getsuga himself.
Kanzen Saimin is an ability that has an area of effect. All of the victims who has seen the releasing of its Shikai will be affected by the spell, regardless of the user's intent.
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.
Once in it's released state, Aizen can activate Kanzen Saimin so quickly and subtlety, it is almost impossible to stall or detect even for a dozen Captain-level fighters on guard.
Aizen only has to show its Released state, not necessarily the act of releasing it. When Aizen showed Baraggan his Zanpakuto, he did not chant its release command, implying that his sword is already in constant state of Shikai.
Kyoka Suigetsu works on all five senses of its victims at all times. However, this might not be limited to just the five senses, as Captain-level fighters who has Reiatsu sense also fall for the hypnosis.
Once the spell of Kanzen Saimin is casted onto the target, it will stay activated even when both the user and Zanpakuto is not around.
As a Captain, Aizen can even create automatic natural behaviour without his active control. He was able to keep members of Central 46 appear living without his own presence.
Aizen is able to use Kanzen Saimin so effectively, he seems like he could bend his victims to his own will.
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.
Aizen can only activate Kanzen Saimin when he physically touches the blade. However, this restriction is also removed when he absorbed the Zanpakuto.
Kanzen Saimin can only work on people who has visually seen the Shikai state of Kyoka Suigetsu. This means that blind people, such as Tosen, will not be susceptible by Kanzen Saimin, even though it affects all five senses of its victims.
Seeing its Shikai will not make you susceptible to Kanzen Saimin if the spell is being activated when being seen. In such cases Aizen needs to deactivate Kanzen Saimin to put the victim under the spell, then activate Kanzen Saimin again.
Ichigo did see Kyoka Suigetsu in its released state in both Soul Society arc and Fake Karakura Town arc, but both times Kanzen Saimin was activated.
Even though he only has to show its released state and not necessarily the act of releasing its Shikai, it seems like he has to reveal the entire Zanpakuto in order to work. This is likely why he gone through the trouble of gathering all the lieutenants to put them in the spell.
When Aizen was still a vice-captain, Aizen stated that even his Kanzen Saimin can be seen through when given enough attention by observing strange behavior from disguising as someone else.
However, this limitation seem to be bypassed after Aizen became more powerful as a captain.
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 had spotted any flaw.
The only way to escape it after being put under the spell, according to Gin, is to touch the blade before Kanzen Saimin is activated.
This method does not work when Kanzen Saimin is activated, as seen from the Unohana example. After Aizen absorbed the Zanpakuto in his final evolution, there are no blade left to be touched and hence this vulnerability is removed as well.
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 spell indiscriminately affects all whom has seen its Shikai form, and all victims sees the same 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.