Responses
Spoilers...
It's not confirmed, but I think it's because she's from a "different" world and has a small core? Who knows what exactly is going on with her, but because of that she's sick and not immune to magic? Something like that, but in reality, I don't think it's clearly stated why
Possible spoiler
I also thought that these "monsters" could understand humans and didn't feel threatened by Ruby, so they "befriended" her, and all thanks to the little core?
Have we found out why MC is cool with certain monsters??? Or we still finding out cause I'm still confused on why they're cool with her