Do you ever think about why Nozarashi chose that form to show herself to Kenpachi? Why she decided a teeny pink haired baby would be what won him over? Do you think it was so he could feel companionship? Or was it because she wanted to prove to him he wasn't the monster he thought he was? Or maybe they both were lost and didn't really know who they were and had to find themselves together. (Just me assuming and being curious, I'd love to hear your thoughts dad! :p)
this is an excellent ask thank you so much @yahchiru.
so, i headcanon that nozarashi didn't always have the appearance that we all know and love. back before unohana, the sword's spirit might have looked like something a bit more monstrous. the sword reflects the soul, and back during those days, the only thing going through kenpachi's mind was murder, murder, and more murder. before he found his zanpakuto, he had only been hunting other people for survival, scavenging and occasionally attacking with whatever sharp object he could find, or if he was really desperate, his teeth. however, after he picked up the zanpakuto he took (from a pink-haired shinigami recruit, i headcanon), he actively started killing people for sport, for no other reason than entertainment. he had become a perfect representation of the zaraki district: full of violence purely for violence's sake.
all of that changed after meeting unohana. when he realized what was happening, and how a newfound joy he felt was so close to vanishing, he pretty much had a mental and spiritual breakdown. keep in mind that, as a child, he would have killed the deadliest individual in the history of soul society, so the amount of power he had to hold back was immense. for my interpretation, his fight with unohana and his subsequent subconscious self-imposed limiters fundamentally changed the nature of his soul, and in turn, his zanpakuto. he entered that fight a murderer and left it a warrior.
but we have yet to answer the question: why baby?
and that is a very good question. with the nameless boy's murderous desires all gone and his psyche sort of rebuilding itself with unohana as the template for what he wanted to be, he was a very lost individual. he never had a childhood, only a fight for survival and a subsequent killing spree, so he didn't exactly know what to do at first. i think that nozarashi was unknowingly manifested at some point after the battle. bankai training involves manifesting the spirit of the zanpakuto into the physical world, and since kenpachi was doing it completely unconsciously, she popped up as the one thing that was still him in a sense, the part of him that never really had a chance to exist in the first place. his childhood
i've said it before and i'll say it again, yachiru is the childhood kenpachi never got to have. he discarded his murderous rampages, finding them boring and senseless, and he was so wrapped up in being like unohana that it shaped much of his own identity going forward. so, the only part of him that was truly himself was what nozarashi appeared as: the reflection of his soul that all zanpakuto become.