Don't get me wrong, I totally agree with you here, but I think in this case I don't really mind. I often dislike love triangles just because I'll feel bad for whoever loses if they're written well and a good character.
In this case I don't mind it since we don't see Rui often enough to get attached to her and ontop of being an annoying character, she's a creep. So it's actually kind of nice that she loses here.