Board Thread:Questions and Answers/@comment-5303512-20191025210913/@comment-5398287-20191026032352

The theory I always believed was that Nozarashi took the form of Yachiru in that moment to present Kenpachi with someone he was familiar with so he would accept the immense power boost he was about to receive. It doesn't make any sense for her to have actually been his Zanpakuto spirit, given that she used a completely different Shikai of her own and had been running around being seen by others for like a century beforehand.