User blog comment:Xilinoc/Ch. 668 - BIGGER,FASTER,STRONGER/@comment-1338810-20160331181810

Also, "Silly Goose" is the best name Yachiru has ever come up for any of the characters. Congratulations Zaraki, now you and Nozarashi both have proper names.

It just occurred to me: maybe rather than being Nozarashi or part of Zaraki's powers, Yachiru is some offshoot of Unohana, created/left by her to watch over Zaraki over the years.