Board Thread:General Discussion/@comment-35659646-20180522182736/@comment-38805854-20190314074024

My theory is that she is the shikai form of his actual Zanpakuto. His own Zanpakuto manifested during the slaughter where she first appeared. The details of the slaughter and their meeting are too murky so I won't speculate on the logistics but I can imagine there's no canon rule for someone with a stolen Zanpakuto to not then be able to attain their own.

She and the two other entities all wield weapons that could be described as shikai. My theory about Sanpo Kenju (three step sword beast) is that it has a front beast, Lumpy, middle beast, Yachiru, and rear beast, Boney.

It's confirmed she never calls out her shikai. Also, while she's swinging her sword around Lumpy and Boney are doing their thing while being invisible; They only appear when she explains what's going on and why her attacks don't miss. She later disappears herself leaving behind the "physical" set of clothing she dawned for her role as Vice-Captain.

Through his fight with Unohana he learns the name of his stolen Zanpakuto. When he releases Nozarashi, it's my belief, he is using two shikai simultaneously to destroy the meteorite; Lumpy, Yachiru, and Boney assisting while being invisible.

Then, in his fight with Gerard, he uses the bankai forms of both Nozarashi and Sanpo Kenju. Nozarashi being the slimmed down cleaver and Sanpo Kenju's bankai being the Oni-like appearance.

As to why she never returns, I don't know, maybe for the feels.