Board Thread:Improvements and Issues/@comment-15519592-20130817040010/@comment-1387259-20130817040918

The people in Ichigo's general group of friends all had a hint of reiatsu from being exposed to him, so of course they can all sense spiritual beings.

And furthermore, I am about 95% sure that Aizen intentionally suppressed his power, as he wanted to kill Tatsuki himself to send a message, rather than have her completely erase from existence. from being near him.