I don't get the need to have her prey on Ruka at that age just to be used for this. Not that surprising, I guess. Unfortunate though because it feels like it really downplays what this girl did back then to a younger girl. I understand Ruka being a changed person, that isn't the issue. The tone of it was a bit off, though. I'll just assume the story switched to become much more lighthearted in the years since those earlier chapters. Maybe she just wanted to forget it and move on from what she set up back then as quickly as possible.
Because like many people you're seeing it from the wrong angle, their issue was never about Rin going for a younger girl but because she toyed with her heart
Like, really, mangaka is not writing her as some eeeeevil rapist or whatever and japanese readers are definitely not reading it as such, the story is very clearly framing it as the wrong thing she did is playing with Rin heart not that she went for a younger girl
As a matter of fact do we even know their age? Rin is clearly not some little girl and they're both in the same school so how much their age gap
really is? A year? Two? Maybe three? Cause some act like it was a 20 something going for a teenager when both were young
The reality is that even if Rin is slightly older than Ruka both are still effectively children and most likely very close in age, she's not some evil rapist who forced herself on someone else and left them traumatized because of it
Rin attraction to Ruka and them having sex was never the point or the problem of their relationship but that Rin was Ruka first relationship that ended extremely bad, if Ruka was the same age the story could have gone basically the same way