@dukkface
She reluctantly still likes the MC, but doesn't want the rep of being a man-stealing harlot, even if her ex is seeing an escort. So that's her motive(s). She needs to research and act in a way that will peel all the other girls off of him before she can get him back, while trying to maintain how appealing she is in his eyes. Constantly asking Chizuru to stop seeing him, and confirming with her if she has indeed stopped are all part of this. Mami's been getting fed lie after lie, but she can smell the bullshit. It's incredibly logical, and somewhat ethical. Disgustingly emotion driven though, to a revolting level, I'm sure.
This isn't one of those trick questions that only serial killers can solve that people use to out them. I mean, manga is basically a picture book. It's literally spelled out for us.