I feel sort of silly posting Claude here because he's just a human who doesn't really have a secret aggressive and violent side or anything, but I feel like he's still probably one of the more creepier characters of his respective story and setting.
I love to draw him all prettied up because it's fun, but in-canon several characters note they find Claude's eyes super unsettling and creepy (his eyes are always half-open, very light and his irises are notably smaller than average, giving him a pretty intense stare), to the point many compare his eyes to animal eyes, especially those of a predator. It doesn't help that he's fickle and hard to read so people can never really trust him and have to keep their guards up, because Claude is notorious for getting back at the people he feels have insulted him or otherwise stepped on his toes - he has the intelligence and social influence to really ruin people and their reputation, and in the time and setting he's from reputation is everything to people.
It's so difficult to explain what makes him unsettling because it's more so in the game than in his bio or anything so you just... have to take my word for it... He just carries himself in the way people cant really tell what he'll do next and there is this "otherness" to him people liken to animals rather to humans even if he isn't, like, feral or anything. He just makes people feel like they're in the presence of a creature they don't quite understand and whose actions can't be predicted.