I just discovered this manga today and read the whole thing, along with some comments on the most impactful chapters. It’s funny to see the divided opinions about the characters.
For me, the only decent character of the lot is Nanase. The only thing we can reproach her with is her meaningless fight for the relationship with Yuni even though it’s objectively over. But she’s just an inexperienced high school girl and many people probably made the same type of mistake with their first love. It’s frustrating for the reader, but it’s actually quite realistic. The stuff with her "neglecting" Yuni in the beginning is overblown.
I don’t get how it is possible to defend Yuni. Morally the worst character, with zero accountability. Don’t get me wrong, she works great as a character in the story, and her train of thought is often very well done and interesting to read because very realistic.
Fuu is manipulative and a bit of psychopath. Her backstory helps us understand why and I have empathy for her, but it doesn’t change the fact. Unless I missed it, we still don’t have any explanation as to why she’s "loved" Yuni from the start.
Yuki started out well, but of course after chapter 35 I changed my mind.
Overall Fuu/Yuni are well-matched as a crazy/dysfunctional couple, and if somehow Yuki gets a big redemption arc Yuki/Nanase would be ok. It’s not great but at least they seem to care about each other more than Fuu/Yuni.