Frisk can see Cor's dubiousness. Heck, four days ago, they would've been dubious too. Smiling, Frisk reassures him, saying, "Trust me, he wouldn't hurt a fly. If anything, you're more dangerous to him than he is to you, monsters have a tendency to just kinda... collapse if anybody even punches them while being just an eensy weensy bit tetchy. At least that's how it works on my world. It's a miracle he's survived so long on a world like this, really."