Jason DiLaurentis is Alison's older brother. Like everything involving Ali, he's something of a mystery. He's never actively threatened the little liars, which in Rosewood is rare — however, he used to be a burnt-out stoner with creepy friends like Ian Thomas, and he threw parties so wild that even Alison was scared — and when Alison's scared, you know you're doing something really wrong. Plus, no one really knows what he was doing on the night of Ali's death; he claims he was too stoned to remember almost anything that happened that summer. Convenient excuse, right?
Currently, Jason spends his time sulking around Alison's old house, doing yard work, and throwing away bloody rags. Clearly, we're still not sure if he's shed his shady past.