Board Thread:General Discussion/@comment-4976518-20140808063123/@comment-25457796-20140924062851

Stefan (Shikai - "Rust & Shatter") wrote: 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. a bit wrong here, aizen does not control people directly, he had momo under hypnosis as well, while everyone thought she was aizen, she was probably thinking they were hollows. nothing thus far in the series hints he has an actual bankai, there were multiple times he could have used it to make situations easier but didnt or maybe could not. the argument of him being so strong and not having one can be made, look at zaraki, zaraki in the 1000 year arc with his eye patch still on is at the level aizen was on karakura, how you may ask? none of the sternritters or shinigami could sense him until he visually appeared before gremmy then they knew he was there, the same way they could not sense aizen in karakura town.

this helps the point that yes he can be that strong and not have a bankai. both him and zaraki are freaks of nature, massive reiatsu probably even more than genryusai as he could be sensed but neither of them atm, insane combat and innate ability. so its not far fetched that aizen does not have a bankai, he made the step from lieutenant to captain all he had to do was use his shikai ability and make them think he had a bankai to become a captain.