Board Thread:General Discussion/@comment-24815992-20140619090507/@comment-29325824-20190720151122

Well, I admit my theory has some holes in it. And it's frustrating that is was never properly answered in the manga, but it's the best explanation I've got.

Anyway, due to a quick search I just did, the best I could find was in "Do Know Always Love You", Isane flashing back to the last time she saw Yachiru (Gremmy fight) who (according to the translator) said "Finally Ken-chan can call out to me" before she dissapeared the moment Kenpachi released Nozarashi against Gremmy.