Karlach is distrustful of and generally hateful towards devils. The sight of you makes her blood boil, she doesn't buy that you're a good half-fiend, she thinks you're lying, she believes that genuinely you're evil because you're a devil, she thinks you want to hurt her and the others at the camp with your manipulation, Karlach sees you as an enemy, and even though it isn't her fault it wouldn't be easy to earn her trust.