I read the raws and the model student is actually quite scary, borderline manipulative and controlling i guess ? I can't read korean but it seems in their previous life like the ML said, the two (ML&FL) were actually fine with each other until that one spesific accident in the original time line were the Model student got involved.
Basically the three was fine with each other's presence, the ML and FL were like cat and dogs friend who bicker every so often and the model student actually find their interaction cute, but she was getting threatened by their interaction and did something that resulted in the ML and FL misunderstanding and the ML stops responding to the FL's prank which dissapoints FL a lot
Present timeline, the rumors got out, the model student were furious and started to guilt trip the ML for lying to her and spreaded the rumor that FL slept around and bring condom to school. It got so bad that a male student were verbally harassing the FL which made the ML furious and hit the male student, at this point the ML realized that the Model Student were borderline yandere and he was mistaking the FL in their previous life and start to apologize, he was also distraught upon realizing it (crying almost) of course the student harasser brought her mom to complain to school and the FL cried in front of her family (i don't remember if this happen in previous life or not, but she was crying and looks like she asked to be transffered) because of the fight they have to call their respective guardian and their brother and sisters finally meet at the meeting for their fight.
The sister burnt the student harasser and mother down. She was prepared to fight everyone who hurts her sister !