Sōsuke Aizen (藍染 惣右介 Aizen Sōsuke ) is the captain of the 5th Division when first introduced, but then he betrays Soul Society and becomes the main antagonist of the series. His former lieutenant is Momo Hinamori. Aizen is a soft-spoken intellectual, accented by glasses during his time as a Soul Reaper captain. While he appears to be very kind to his subordinates, he cares very little for them in truth. According to Aizen himself, he is not bound by morals and will therefore stop at nothing to further his ambitions, including manipulating or killing anyone if he deems it necessary. Aizen is an extremely potent combatant, well above most captain-level Soul Reapers. He goes on to state that the only way for him to advance beyond that would be to gain and master hollow powers. His mastery of the regular skills are such that he is able to stop fully released zanpakutō, includingIchigo Kurosaki's bankai, with one finger, and he can defeat several captain-level opponents at once. A hint to his actual level of spirit power is given when he turns Wonderweiss Margera into an arrancar. When preparing to activate the Hōgyoku, he states that despite its half-awakened state, it can be fully activated when temporarily fusing with someone with twice as much spirit power as a captain-level Soul Reaper, referring to himself. The force of Aizen's spiritual pressure is also great enough to bring the 6th Espada, Grimmjow Jeagerjaques, to his knees without any visible effort from Aizen himself.
Aizen's zanpakutō is Kyōka Suigetsu (鏡花水月, literally "mirror flower water moon"). In its sealed form, it looks like a normal katana. Its shikai is both activated and deactivated with the command "shatter" (砕けろ kudakero ). The phrase kyōka suigetsu suggests its illusory properties; the moon reflected in water and a flower in a mirror are things that can be seen but not held. It is a Chinese idiom for seeing and desiring something that is a dream that cannot be easily grasped in hand, teaching people about temptations.
Kyōka Suigetsu's shikai has the ability to place anyone who sees it under a state of "Perfect Hypnosis" (完全催眠 kanzen saimin ), after which Aizen can make them see, hear, and feel whatever he wants. According to Aizen, the hypnosis is unbreakable, even if the target is aware that they are hypnotized. The only way to avoid the effect is to not see Kyōka Suigetsu's shikai, making the blind Kaname Tōsen immune to its effects. The effect seems to be permanent, or at the very least long-lasting, as Aizen uses it on nearly all of the lieutenants and captains in the Gotei 13 during a ceremonial release of his zanpakutō years before faking his own death. While the illusions Kyōka Suigetsu creates are quite powerful, they are not 100% accurate; Captain Retsu Unohana was able to realize that something was amiss with Aizen's fake corpse, even though she did not realize what until seeing Aizen alive.
Kyōka Suigetsu's hypnotic abilities make it a useful tool in combat as well, as Aizen can quickly create simple illusions to deceive opponents. By creating a decoy of himself, Aizen can hide his own movements and attack with impunity; his opponent, meanwhile, will remain focused on the illusion until it's too late to react. The most prominent example of this is his brief fight against Sajin Komamura, where an illusionary copy of Aizen is seen melting away after the real Aizen has moved close enough to use the devastating Black Coffin kido.