@Comrade-Sandvich i agree partly, because i don't think that Misukana is that fragile, she has a fragile side when it comes down to it, in the sense that she is not really independent and thus she has a level of impotency that limits her options to just fall and cry for help, however she also uses that same fragility as a tool to get men to intervene in her favor, she is one of the characters (and i will even argue, the key one) championing and selling the idea that women ought to be protected by strong men, which is how she convinced the MC to insist on going after sensei, and at the same time she has been too active on her endeavors, because she knows she is fragile and wants to do something about it, we just don't know yet why exactly she is doing all this, and if she wanted to be just an observer, why even be right next to them with her phone pointed at them? she wants to let them know that she knows about what they are doing, she is practically making an statement by just standing there.
On the other hand, we haven't seen the fat girl at all, as far as we know the public break up put a stop to her shenanigans before they got out of hand, and we have hardly seen her again even as a background character, unlike Misukana the fat girl hasn't collected any evidence against the MC, and she was the one that was making their relationship public by openly advertising that she had a boyfriend and that she had befriended his mom, which was true, so saying that they kissed and touched, or that they did even more wont have much weight because albeit short they did go out, did some of those things, she befriended his mom, and then they broke up, and everyone knows, there's no material there for the fat girl to use because she already spread the details.
If the fat girl wanted to blackmail the MC she would have to start inventing stuff (and she would have to get quite creative because she has a reputation for being an easy girl), she would have to start copying Misukana, and create an scenario in which she can get something to blackmail the MC, which would be difficult since they are nothing anymore and on top of that she is not all that smart, it would be easier for her to steal Misukana's phone, but alas she is oblivious to what her "best friend" is and was doing behind her back.
I really want to be wrong about Misukana, and think that we may get something good out of this for once, but she is making it too difficult for me, Misukana says that she wants to help people, and that she does if for the well being of others, but i have yet to see this, for example she got the MC to break with her friend because she wanted to help her friend, but out of all that the fat girl learned nothing and instead got herself publicly humiliated, while at the same time she pushed the MC away from a relationship between teens to one with his teacher by selling him ideas to inflate his ego as someone that can protect others while the guy can't even protect himself, all of this while using underhanded methods.