It's more like one character getting buffed.
It's pretty clear she has a thing for Val, yeah. Tbh, characters like her and Carrie (the girl Linda mentions during the SMS talk) are meant just as excuses for 3somes (which is one fetish I really enjoy). You can avoid content with her, yeah.
Just don't expect Val to never mention her again or to her never show up on the game again.
The way I write my story, the girls don't exist only to be MC's cocksleeves. They have lives, friends and things they like to do by themselves. If you don't really feel comfortable with it, well... I guess the game isn't really for you then.
That being said, this is a harem game and NTR/cheating is definitely one of the things I can assure you will never be part of it. So don't worry, nobody will cheat on MC or do stuff behind his back.