Feels like my off the cuff theory from last chapter might be onto something?
I'll cut & paste it here again:
Ch.2 P.19, the charm said:
Whoever you do not choose will disappear.
...As that is her wish
If we look from the overarching theme/moral of the series that this is Karin learning to accept all the facets of herself through seeing that Yuu, the man she loves, also loves and accepts every and all aspects of herself. The message here is Karin doesn't accept parts of herself and likely believe Yuu will hate some aspects of her; in the mix of Yuu wanting to see her and the charm said it thinks the more the merrier, she did has a say in this shenanigan that she wants the part Yuu doesn't want of her to disappear. After that we also learn from Kumaka arc that it's not only Yuu who has to accept that aspect of her, but Karin herself will have to accept it, too, otherwise she'll disappear. Hence, I reach my conclusion above that the core of this story is Karin learning to accept herself through seeing Yuu accepts and loves all the facets of her.
EDIT
Giving it more thoughts about the magnitude of her wish hinted at in this chapter, maybe her wish doesn't have anything to do with Yuu's and his charm's shenanigan, but purely she went through some traumatic event, doesn't accept herself/hate herself, and probably wish to disappear thinking Yuu will hate what she has become, but it now clashes with Yuu's wish, probably.
Whatever it is, I think the overarching theme still stands that she will have to learn to accept/re-learn to love herself through Yuu.