User blog comment:Xilinoc/Ch. 668 - BIGGER,FASTER,STRONGER/@comment-1268838-20160331160224/@comment-1268838-20160331172724

I would take any gender pronouns, especially those spoken by one person in regards to another, with a grain of salt. It would not matter either way, as this arc established that his Zanpakuto has not spoken to him until this arc... with the exception of whatever possible weirdness is going on.

Anything with regards to Yachiru somehow being a Shinigami is something that still needs to be explained, obviously.

If Yachiru ends up being his Zanpakuto spirit, Zaraki is clearly unaware of the fact. And yet, he would have communicated with her for some time now. As I said before, he seems to be doing things backwards.

And again, if she is the spirit, she has already been materialized for more than 10 years. That in itself is the first step to achieving Bankai, and the one that Uraraha's invention allows a Shinigami to bypass. As far as subjugation goes, who knows? This is apparently an exceptional case.

At this point, unless there is some other reason for Yachiru's vanishing act, it's the only obvious answer. I'm not even going to go into the quality of the writing until Kubo explains things himself. I do not expect anyone to be pleased by the explanation, but there you go.