Man I am so lost as to what is happening.
Is she dead?
Or is it some Silent Hill esque thing of her emotions, feeling and fears manifesting on the real world?
she's not dead.
When Karin moved away from Yuu and into middle school, she lost her sense of self as she grew up, for reasons that can happen to anyone.
But I suspect Yuu was her grounding force, and she yearned for the simpler times of childhood that she had, alongside him, and wished for things to go back to that point when she could just be a carefree child without worries or responsibilities, alongside Yuu.
And the crux of the issue is she wanted that return to simplicity and to forget all her worries and anxieties and all the "voices that contradict one another" that are born from that - and the Charm granted her wish
after taking her literally.
Hence her splitting.
And so yuu met all of Karin's various "sides", and was supposed to pick the one version of Karin that she would fully manifest as, so she could go back to being untroubled and could just be with Yuu again and be happy.
But Yuu, loves all of Karin, and couldn't pick "just one part of her". Because that's who he loves. And so
his wish of seeing her again conflicted with
her wish of wanting things simplified at Yuu's hand, and the manga up to this point is the result.
But when that didn't work out, and Yuu accepted
all of Karin, all her memories came back, she remembered all the pain she'd been in, and the total Karin scope collapsed, resetting back to what would "grant" her wish most literally - reverting her to her child self from a decade before.
And with that, all the aspects of Karin that were split off and manifested are consigned to be destroyed and erased, so that Karin's wish can come true. And Yuu now has to keep that from happening, so that he doesn't lose the Karin he loves, which is the whole of the sum of all the Karins we've met.
Or at least that's my take on it.