I'll stick with the sadist because she doesn't really give off any vibes about being a real sadist.
It feels like she read a book about sadism and thought it might be fun to try it out.
Also to find out who Cinderella is, why didn't the MC look at the size of the panties?
Each girl is different in size and shape: Sadist can't wear Masochist panties and vice versus.
huh.. now it feels so cliche.. after this the senpai is innocent again and he will become a slave, and he will try to be gay with his friend, and the new girl approaching.