User blog comment:Godisme/Deicide 17/@comment-1935515-20100807074026

Even if he's not a Hollow, he might still have things in common with them... ... I mean besides the hole/window in his chest, the black irises, the ability to grow stronger from eating people, organic morph rather than weapon morph, possibly a weird skin colour... (he never wore a mask, though, I'll give you that!)

Also, Gin could've by accident invoked some Hollow-ness - against Aizen's will - when he gave him a hole in the chest. That might explain why Aizen got so pissed right before his transformation.

And besides, you guys believe Aizen can't be a Hollow BECAUSE HE SAID SO?