User blog comment:Xilinoc/Ch. 668 - BIGGER,FASTER,STRONGER/@comment-1970092-20160331203556/@comment-3423279-20160404060413

If a Zanpakutou spirit learns about its limits when a Shinigami does then why does Jinzen involve learning about the sword spirit's abilities? If the blade only knows what the Shinigami does than this wouldn't work as you would be trying to learn something who has only your knowledge.

Zanpakutou Spirits possibly know what their Shikai and Bankai is long before a Shinigami.

A Shinigami seems to need to be "worthy" in some manner to hold a Bankai.

I mean look at Renji, even though he attained Bankai he still wasn't worthy of its full power in his Zanpakutou's eyes, the spirit of ones blade seems to have more control over the release more than one would think.

If Yachiru is indeed Kenpachi's Zanpakutou spirit then she might view him as worthy of Bankai at this point.