Hmmmmmm
Rin seems a lot less toxic in her feelings than I feared she might be after last chapter.
Mayu for one seems to at some level understand Rin's feelings what with the kiss back after she confessed.
But the problem is that Mayu wants to keep Rin at a distance for protection against the supernatural entities, but she doesn't explain that to Rin at all.
So all she sees is Mayu being taken away from her by some senpai she barely knows who gives off rancid vibes with the level of control she exercises over Mayu, especially since Rin doesn't know about the mother roleplay.
Frankly while you could interpret that last panel as yandere-like jealousy, it could also be concern about Mayu entering a coercive relationship again and Rin being unable to help again.
Mayu kissing Rin is Mayu mirroring Komachi kissing her in chapter 3. Mayu was spiraling during their lunch on the steps up to the rooftop, and Komachi leaned forward, kissed her, and said the lines that Mayu borrows here with Rin. It worked out in getting Mayu to calm down from her panic; so when she saw Rin doing the same thing, she copied the move.
I think there's an argument that Mayu's not completely dense when it comes to Rin's feelings or their significance, but I would also argue that Mayu's not in a place where the type of relationship that Rin wants, could be achieved. Rin's not wrong for seeking one out, but I'm not at all surprised that Mayu has pulled back, and not solely because of the supernatural horrors that are after her, and a desire to see Rin protected from that. Mayu isn't in a place where that sort of connection would be healthy, at all....it's why Komachi is "mom", and not "lover", in the manner Rin desires. Mayu doesn't need a girlfriend, she needs a protector, and independent of Rin (or anyone else) not knowing the extent to which Mayu's life is now Eldritch-Coded, the fact remains that Komachi is singularly equipped to actually provide protection.
And Komachi also kinda covered the whole "not telling people" in chapter 3; not only did she say Mayu should lie to the police investigating the house fire, but she herself hasn't told Mayu
everything going on, because she questions whether Mayu is ready to know the full extent, or if it would simply break her. Mayu protecting Rin is again a sort of mirroring of that; it's less overtly maternal and more "I'm looking out for someone special", but I do like the parallels drawn, even if some of the characters aren't aware of it.
I could be jumping the gun on thinking Rin's going to go to a dark place, and she could very well simply be embodying the "concerned love one" who also fills the "no-powers ally" slot. But the way she describes wanting to have Mayu to herself, and the way she was so proud of being the only one who would dare approach her when they were kids and the rest of their class had written Mayu off,
does lend toward a route that sees Rin begin to spiral, now that she's suddenly seeing "her spot" being usurped.
We're definitely meant to sympathize with her at minimum, sure--as Komachi remains an enigmatic figure who might not actually have Mayu's best interests at heart. But I will not be surprised if Rin gets some sort of Turn at some point, and while I doubt she'll become an actual villain to Mayu (and I definitely hope she doesn't die), I could easily see her becoming Komachi's adversary, in particular, and grow more aggressive in attempting to "save Mayu" as she sees it.