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

The fact that she disappears when Zaraki releases his Shikai, and reappears to activate his Bankai before vanishing again (and IIRC only Zaraki could see her) indicates 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.