Board Thread:General Discussion/@comment-4976518-20140808063123/@comment-8163713-20180113023654

Aizen's bankai is kyouka suigetsu. He never once confirmed it was a shikai, he called it a release, but didn't say which one. You get the confirmation in the final arc when Yhwach states that in the future all bankai have broken and Aizen's sword cracks while KS is in effect. Suigetsu is Aizen's Zanpaktou, it mirrors Isshin's Engetsu. The kanji on both have dual meanings, phonetically and written. And their respective elements oppose each other.