Interesting tidbit about Makino's past. It doesn't really change the general inexcusability of her behavior, but it's nice to know that it didn't come out of nowhere or from her family, and that the "inferiority compared to her sister" of the first chapter was as fake as it looked. If she's been ostracized for her whole high school life, it's hardly surprising that she didn't have much motivation to get into college afterward.
Kinda ironic that she disliked being judged from her looks, but then did exactly that, and only learned about Satou on a personal level over time. Yet again, very fitting for her personality, and the usual story of "I've been hurt, so I'll make sure no one else goes through this because of me" is a naively optimistic take. Makes sense that treating someone badly leads them to treating others badly, too.
Not-so-useless manga character confessing in public : happy fangirling noises
Not-so-useless manga author confessing in credits page : confused fangirling noises
Your comment confuses me, Scummy very much have been repeatedly called inferior to her sister by her parents. The fact that her social life was a disaster too doesn't mean it has to be one or the other. There hasn't been any judging someone by their looks, Scummy was desired by the boys for her looks, she was hated by the girls for the attention she was getting, she grew fond of Mashiro because she functioned as a safe space for her and concequently fell in love with her because she is lonely and suddenly had someone she was intimate and relaxed with.
Scummy is judgemental and not socially adept due to her hard life which means that she isn't acing supporting Mashiro, but it's not like you would ever have expected her to be at all smooth at that role. Her heart is in the right place.