Oh... OH!! Fuck.
I think I understand what's going on. That girl at the end isn't related to Fushi. That's not why she's regenerating.
Based on context from this chapter, it's clear that she's a victim of ijime (bullying, specifically, the hardcore japanese type). I'm guessing that she committed suicide and that her body is now controlled by a Knocker. Maybe these Knockers who have full control of human bodies are able to regenerate like this.
Also, since the issue of it being difficult to find spirits of the dead who are still hanging around right now was brought up, I can think of two general possibilities for where this is going and why it was brought up in this chapter in particular:
1- Maybe these Knockers can affect spirits to some extent, but I hope not since that would be the worst case scenario.
2- Maybe this combined with the focus on the girl at the end of this chapter is there to imply that her spirit is going to play a role.
or maybe I'm just reaching, but things aren't usually put in a story without there being a narrative purpose for them. Sometimes it happens, but normally there's a reason.