Board Thread:General Discussion/@comment-24189673-20131120064700/@comment-25227771-20140918062715

Just throwing this out there but except for Bushogoma, all of Katen Kyokotsu's games end with -oni (鬼, demon) e.g.: Irooni (Color Demon), so perhaps his Bankai is something about demons, hence why Ukitake didn't want him to release it perhaps due to its frightening appearance.