Ugh... that brat is absolutely insufferable. This is beyond annoying.
Throw it into traffic... or in front of a train.... or burn it to death... or find a meat-grinder. Hells, it doesn't matter. Just kill the kid. Reverse up and sit on her the next time she grabs the butt. That should do it.
I swear this mangaka specialises in making intolerably annoying characters and using them to ruin everything they make.
... Speaking of which, that "senpai" looks rather like Asuna Ashitaba... but with smaller boobs.