Board Thread:Administrative Requests/@comment-26830922-20170103215204/@comment-33343489-20190810075736

The only thing that I think the manga has outright said is when Yachiru tells him that if he uses HER properly there is no enemy he can’t be and then says that power is called Bankai.

So I’d argue that that is direct confirmation from Yachiru herself. Plus she not only disappeared right when he went Shikai but she appeared again for the last time when she released his Bankai. So it was stated by her directly, but I get that it’s a weird situation of how she exists with her own zanpakuto spirits and Shikai