Board Thread:General Discussion/@comment-4976518-20140808063123/@comment-5898946-20180523015949

Given that Aizen is at such a peak level, it's unimaginable that he doesn't have Bankai. However, given how powerful his Shikai is, he really has no need to use it. We've seen that many Bankai are draining on the user. Also, it could be a Bankai that exerts a heavy price, such as Sui-Feng's which is massively taxing on her body to the point where it's difficult for her to fire more than once every three days. If Aizen's Bankai has a similar restriction due to the sheer power output causing stress on his body, it would make sense that he would never (or at least, very rarely) use his Bankai, because there would be no real reason for him to use it - his Shikai is more than enough as it is.