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

I don't know if it's been mentioned here yet, but:

1. Bankai has been stated to require the user to be able to materialize their Zanpakuto spirit into the world. If Yachiru is his spirit, she's been manifested for centuries. And they already have a perfect relationship, so why wouldn't she be able to give him Bankai? It seems like his only hold up was not knowing his Shikai.

2. Yachiru's Shikai, if she is his spirit, could just be a technique of his Bankai.

Thing is we don't know yet, and it will be explained next chapter or after. Don't be so quick to write off Kubo. Personally, I'm all for her being his Spirit as it just makes all the sense to me~