I don't think the manga shows how important Charlotte is to Slow. They don't depict her as a girl with the standard qualities - cute, kind, etc. All is see is a stereotypical klutz who has a dark past slapped on for depth. If you want the audience to root for Slow you should make them also want to protect Charlotte, but I don't feel anything. Abusive tsunderes are IMO the worst cliche of them all, but since the premise of the story is not your everyday isekai, I can understand. I'm actually rooting for the kouhai, since she actually did something cute.