Based on Fuuko's narration at the end, it seems her complicated feelings are rooted in the toxic, defeatist mindset Mashiro has instilled in her: that she is destined never to be anyone's first, should settle for what Mashiro provides, and that they are, in some sense, kindred spirits because of this. It's a very nasty and unhealthy idea, and one that Fuuko likely clings to whenever things don't go her way (as they did recently with Yuni).
I don't think Fuuko has any genuine romantic feelings for Mashiro beyond that, and Mashiro's strength comes from keeping Fuuko isolated and depressed. She's like a parasite taking an opportunity. This is not sustainable and not the endgame for Fuuko (I'm sure). It also potentially positions Yuni as Fuuko's savior, since she can show Fuuko that she can be someone's number 1. If the author intends for Fuuko to have a good ending, this chapter makes me more confident that she and Yuni are endgame.
This entire arc into Fuuko's backstory and the dynamic between her and Mashiro feels like real intent on the author's part to make Fuuko sympathetic. If the girl who was raped by her senpai as an adolescent isn't given a narrative upswing, I think it reasonable to assume that none of the characters introduced are getting anything resembling happiness come the final chapter.
I'm not convinced that Yuni represents end-goal happiness for Fuuko either, but I do think the two of them being together gives them both the best shot possible at figuring out what a healthy, fulfilling relationship looks like. Nanase is too self-absorbed to give Yuni what she needs, and Yuni is too needy and insular for any long-term "good" relationship between them to work. Her propensity for falling 100% into her partner and wanting constant contact & attention makes me wonder about how co-dependent a relationship with Fuuko would look like, but it could also simply be a case of them both needing to grow up a bit more, as well.
Time will tell, though, if that ends up being all they need,
and whether they'll even be given the chance to figure it out, considering both Fuuko and Yuni have others gunning for them (in various ways). If I were a betting person, I'd say that the endgame would be Yuni & Fuuko, and Nanase & Yuka, and Mashiro with...I guess no one? I do think she's got some sort of history with that one long-haired sensei we first see in chapter 33 (p.17), but I honestly think she's just incapable of meaningful romantic/intimate connection, and thus she's wholly here as an antagonistic force acting upon Fuuko, and representing the toxic mindset she needs to break away from.
But I'm not sure I'd call Fuuko and Mashiro kindred spirits, if only because Fuuko seems to have
become this way as a direct result of Mashiro influencing (and really, outright manipulating/grooming/what-have-you) from an early age. She longs for love and connection, and is now in the mindset of simply chasing 2nd place--to me, "kindred spirit" gives the impression they're inherently alike, in a natural way...and what's been done to put Fuuko in this mindset is anything but.
