Board Thread:Questions and Answers/@comment-931054-20180417134051/@comment-25192437-20180716040008

The fact that "Yachiru" disappears when Zaraki releases his Shikai, and reappears to activate his Bankai - telling him he needs to learn how to properly use her power - before vanishing again (and IIRC only Zaraki could see her at that moment) strongly implies that she is his Zanpakuto Spirit. I've heard that Japanese writers often expect their readers to be able to put two-and-two together on their own regarding details like this, so the chances of explicit confirmation of this isn't likely - unless we get a new databook or something.