She laughed like someone who didn't know better. If Snape wasn't -- well, himself -- he might well have been disarmed by such a laugh. But he was, and so he wasn't.
Darcy spoke like a woman with humor and confidence. She was American and she wore muggle clothing comfortably, which only really meant that she wasn't from his world, and probably didn't have any magic of her own (although this he could not say for certain, since magic was more varied than one might expect). Most of these things (but not all) were reasons Snape might not like her but he was giving it time. Surely he'd find a real reason soon enough.
"Severus," he said after a beat because no, he did not want to be called Roomie ever again really. He huffed out a sigh, put upon, because he did know what humor was, even if he kept his own checked. He didn't think she was particularly funny, but he got the feeling she thought so. "Are you always like this?"