I saw a thread earlier pointing out that it doesn't seem that Yachiru has aged over the years and it reminded me of something I though of a while ago.
Stark the number one espada and Lilinette were actually one soul split into two souls to make it possible for them to live around someone without killing them right? Well my theory is that Kenpachi was the soul reaper version of that.
He might of split his soul into two (well his zanpuktos true form) into something else without knowning he did it. This might be why he hasn't heard his sword talk to him in ages, maybe he has been talking to his sword the whole time in Yachiru.
Anyway, just thought I would share my random ass theory, kinda stoned. Feel free to prove me wrong!