User blog comment:Xilinoc/Ch. 668 - BIGGER,FASTER,STRONGER/@comment-957629-20160331154647/@comment-26091912-20160404092019

No matter how you slice it, her being his Zanpakuto spirit makes no sense at all.

Zanpakuto spirits, as far as we know, reside in their wielder's inner world unless they're 'pulled' into the real world. In which case, Yachiru meeting Kenpachi for the first time would also be the first time she's been summoned (albeit unconsciously) into the real world. Therefore it wouldn't be possible for her to have 'had a life' prior to that, being from District 79 and whatnot. There's also no way he could've rescued her, if she was supposedly with him the entire time, as his blade.

Even in this arc, it's been made abundantly clear that she couldn't be his Zanpakuto: Kenpachi hearing his sword's voice for the first time, introducing itself to him, etc. And the fact that Yachiru has her own Zanpakuto with a completely different name and ability altogether.

You gotta admit, nothing about it adds up.