Even in half demon hunter clothes, Clary thought, he looked like the kind of boy who'd come over your house to pick you up for a date and be polite to your parents and nice to your pets.Jace on the other hand, looked like the kind of boy who'd come over your house and burn it down just for kicks.

Clary is comparing two characters: Jace, who is a half demon hunter, and another boy. She visualizes the other boy as being polite to her parents and nice to her pets when he comes to pick her up for a date, whereas she pictures Jace as being the type of person who would come over to her house and burn it down for fun.