And in Ch. 6, she literally gropes FL's boobs out of nowhere while FL was wearing a pretty and short dress she personally picked out for her. I know it could also just be her trying to assert her dominance over FL, but that random boob grope...
And she said she knew FL would need her help to strip off the dress that's why she followed her inside the dressing room... but she was the one who picked that tight dress for FL in the first place...
it was really fucking weird when she just suddenly groped the fl’s boobs also yeah I feel like there’s history with the both of them, there’s obviously an ulterior motives as to why the antagonist girl is doing this, prob out of fun or maybe there really is something deeper to it. Would probably be cool if the plot twist was that she was actually inlove with the FL and loves putting the FL down as some sort kink

Re-reading and low-key am seeing the antagonist girl having toxic lesbian vibes for FL. Not sure if this translation in Ch. 12 is accurate but:
"I just told her that her boyfriend is a cheating piece of shit stringing his innocent girlfriend along
Fuck... what's with that look?
This is why i should've been born a guy.
Your life must be so fucking easy and fun, huh?"
If it is accurate, it almost sounds like the person she's trying get rid of is FL's boyfriend. Like she had multiple affairs with him just so he'll get dumped by FL lol. Wild if that is the case.