Board Thread:General Discussion/@comment-26329270-20150420071826/@comment-5398287-20150420082919

We don't know exactly what happened with the 2nd Kenpachi, though it seems most likely that Unohana just left her position and was replaced since everyone alive around that time remembers her real nature (meaning she didn't fake her death or anything), but apparently the tradition of killing the current Kenpachi to become the next one started with the 2nd one, possibly at Unohana's request: after all, she basically engineered her death at Kenpachi's hands, both because she wanted to unlock his full power and because she felt there can only be one Kenpachi and that the title would properly be passed on this way.