Wow. Opinion of Sadist Bitch-san is pretty divided.
Personally, I hate her, and I love that I do. She was teased as a love rival character, but got swerved from that hard for something more akin to an antagonist. More importantly, she's stimulating a lot of potentially nice development in the whole cast.
Stimulating negative feelings in Unoki might cause something interesting to happen, fearlessly instigating Toramaru might get something interesting relationship-wise, going for the boobage might push Yutaka and Matsuri's relationship somewhere, and her being a generally agreed-upon shitty person in general opens up some interesting interactions between a cast of otherwise nice people. And Matsuri.