Board Thread:Questions and Answers/@comment-27015288-20160119152648/@comment-8986886-20160121155125

Salubri wrote: 1.) The problem presented here is when Yamamoto caught Aizen in his trap. He specifically points out that he knows Aizen is the real thing and not an illusion cause he can sense the reiatsu from his zanpakuto.

When Gin turned on Aizen, he said that the only way to cancel out Kyoka Suigetsu's illusion is to take hold of the blade before the illusion takes full effect, so that would explain it. Besides, Yamamoto could be pretty arrogant at times and even if he claimed to be able to see through Aizen's illusions doesn't make it a fact.

Salubri wrote: 2.) There is a possibility that Aizen keeps his reiatsu hidden/close to his person, that would make the most sense given the nature of his ability is subterfuge and espionage. That would also account for the fact that he can move around undetected during the SS arc.

It's more likely that he used a Reiatsu-concealing coat to move around unnoticed, like the one Kisuke had invented a hundred years earlier. Aizen, Gin and Kaname were seen using such coats during the White incident.

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When the Shinigami intercepted Aizen in FKT, Omaeda made a comment on how Aizen's Reiatsu was monstrous, just like he remembered it. I also doubt that Shinigami can completely hide their Reiatsu from opponents during combat. For example, when Isshin fought post-evolution Aizen, he said that it was creepy how he couldn't sense Aizen's Reiatsu. So, how was pre-evolution Aizen able to trick the Shinigami into fighting Momo, if he was unable to affect their Reiatsu senses? Even if he had come up with some ingenious way to hide his own Reiatsu, how was he able to hide Momo's? Even Kira, who was healing Momo before this happened, never suspected anything when Aizen took her place.