It is absolutely Rin.
Looking back at chapter 19 and this (and at chapter 39 when she first (re)appears), the hair and the eyes are way too similar to be coincidence or some other overbearing woman who'd just saunter in, that Ruka would immediately recognize and have such a hostile reaction toward.
And yes, you are correct (in your second comment responding to Picquart) about Rin being the reason she's so cold and distant toward most people, and why she can easily get physical with girls but simply goes through them without building meaningful connections or bonds. She had a crush on Rin, who fully used Ruka--and because of that "one-sided specialness", Ruka felt so betrayed that she turned her heart off to emotional connections, in effect.
Suuna's the first one to get past her walls, and it's why Ruka pursues her so much. Suuna represents the first time Ruka's found "something special" that's actually two-way, and so she's putting all her energy into Suuna and swearing off her old habits and sex-friends.
I will reassess my original statement about chances for a "triangle" and say that it's much less likely that it becomes a real thing.
But, Rin represents the start of Ruka's "fall" into the empty & cold point of her character arc. Rin was responsible for disillusioning Ruka to the concept of romance, after being used by Rin and then discarded.
Rin, for her part, likely has no real desire to be with Ruka in the manner that Suuna does--but I will double down on the malicious intent prediction, because her personality seems to revolve around manipulation, coercion, and exploitation (at least where Ruka's concerned). She forced herself on Ruka, then "played with her" until it was convenient to simply walk away, and it fucked with Ruka for years afterward.
So now, she sees Ruka doing "much better", by all accounts, and it's all because of Suuna. She's probably intrigued as to how Ruka's turned the corner like this, and is coming back to screw with her again. Maybe she has more meaningful motives, but it reads like she's just malicious for the sake of it, and it's no deeper than her simply enjoying the act of messing with Ruka.
So it'll depend on how Suuna responds to these revelations, assuming she gets wind of what Rin was to Ruka, and what she did to Ruka. Because in a way, Suuna has also forced herself onto Ruka, but it was not in the same manner as Rin. And that being the key point, because everything else about Suuna is what made Ruka fall for her.
Suuna represents Ruka's growth and ability to actually fall in love, that Rin took from her. Now Ruka's Past, and Ruka's Present/Future, are confronting one another, and we'll have to see how it all shakes out.
