Board Thread:General Discussion/@comment-4976518-20140808063123/@comment-8986886-20140808095543

Considering Aizen's potential and hunger for power, and the fact that he has mastered just about every other aspect of what a Shinigami should know, I can't imagine him not having achieved Bankai. Since Aizen should be able to mask his Bankai release, he might have used it without anyone realizing it. For example, when he was able to control Momo in FKT, and made her fight the other Shinigami, even though she had been critically wounded by Ayon earlier. With a power such as complete hypnosis is would be very difficult to tell the difference between Shikai and Bankai.