Either that or an ending where NOBODY is happy...
I dunno...in spite of how outrageously melodramatic and toxic the four main characters (plus Mashiro) have been throughout this series to one degree or another, there are actual through-lines that connect between them that also satisfy some of their primary narrative drives. Because ultimately, Nanase ending up with Yuki, and Yuni ending up with Fuuko, makes a fair bit of sense that resolves a lot of current plot obstacles for each of them.
This is my personal read on the characters, but —
Yuni needs a partner who will prioritize her and always put her first, and I would argue she would also extend the same sort of dedication to someone meeting her on that wavelength. She has a strong relationship with her single mother, who was often partly absent due to needing to provide for the two of them. That lead to Yuni feeling attention-starved, which is why she fell in with Fuuko in the first place: Nanase just wouldn't pay attention to her, or
enough attention to her at the times when it actually really mattered.
Fuuko needs a similar sort of attachment-oriented bond; she grew up in a very strict household where she could never be herself, repressing her feelings of unrequited love and watching the person she fell for be "taken" from her. She was then captured in Mashiro's web, who twisted those feelings of belonging and intimacy, inverting the desire for affection and affirmation into toxic versions of themselves, while left feeling invisible to those around her. She saw a parallel in Yuni, which lead to her reaching out and spawning the first major narrative upset. Yuni then slowly becomes a potential "better" partner and lover, as she seeks genuine comfort and affection from Fuuko, which is something she is starved of via her family and Mashiro.
They're arguably very codependent when they're together, but there's at least a future for them with the possibility of helping one another grow through those issues, potentially with outside support (Yuni's mom, perhaps). They're still young; 16 years old is a fucked up time to be alive without all of this messy toxicity, but not everyone stays the same in five years' time, assuming an understanding of things needing to change and some applied effort.
I don't know if they'll get a
happy ending, per se, sure. Could be a "they walk off into the sunset, but an epilogue shows them separated" or something. But they are a valid endgame option for the other, because they click in the ways that matter to them.
On the other side - Nanase is very extroverted and has things that are important to her, and she does not want to give up time or attention to them if she doesn't have to. She knows what she wants, but that also means her focus is drawn away from everything else—which lead to Yuni's increasing frustration and eventual infidelity and the complete mess that transpired.
Yuki, for her part, is apparently more than fine with "sharing" Nanase with Nanase's interests, because they actually have those in common to some degree. Her overarching desire is to have Nanase for herself, but not in the exact same "look at me and only me" manner that Yuni had. She doesn't crave validation and attention and affirmation, but she
does want to be by Nanase's side as much as possible.
A prime example: Yuki would have been fine with the camping trip including the volleyball team; she just wanted to be by Nanase's side the whole time. Yuni was mad the entire time because she thought it would be just her and Nanase, which is what
she wanted most from their relationship.
Yuki's very possessive and more than a little manipulative in her own way (a quality she shares with Fuuko, which is why they're mirrors within the narrative as it pertains to the Main Two), but the exact texture of her desire for Nanase's attention is distinct from Yuni, and thus their established relationship and the rapport they share coupled with their interests and ability to engage with them together, means they could end up in a reasonably sound relationship.
And like the other two, there's every chance it couldn't last; if Yuki gets too jealous or insecure, she could fall into a similar pattern as Yuni did. But she's already equipped to weather it better because of the history that she and Nanase share.
Mashiro ....I dunno. For a split second I thought there was something between her and that bespectacle'd sensei from her own high school back in chapter 33 (page 17 in the last panels, in particular), but I think she might be more well-developed plot device for Fuuko and Yuni, than someone who is intended to get their own personal resolution to their story. There's really nothing there for her, that I could see during the big flashback stuff of the previous volume; she's just toxic and manipulative and
potentially aromantic (but I'm not putting my name on that whatsoever) but more likely just incapable of love and affection, but there's no impetus to her character like there is for the other four that requires resolution for her own sake. That, and she was introduced/fleshed out so late in comparison that she feels more "conventional antagonist" than anything else, to me (like Final Boss material).
That's all just my thoughts on where this could potentially end up. I don't pretend to know or to even be confident, but there's at least some argument that they could each see an ending that could be read as "happy", or at minimum a definite narrative conclusion that satisfies what each of their characters are seeking and desiring, with the potential that each could end up in a better place than they started.
