Waitress Marlene

Waitress "Marlene" is the waitress at the restaurent/diner where A and Anne Sullivan met in Season 2 episode Over My Dead Body. She doesn't appear until the season finale of Season 3, A Dangerous Game (confirmed by Marlene King on Twitter).

Season 2
In "Over My Dead Body", after A and Dr. Sullivan met and Dr. Sullivan just left she asked A if she wanted some pie.

Season 3
In "A Dangerous Game", she possibly plays a vital role. Maybe the Liars track her down, and she tells them something about "A". We do know Dr. Sullivan is back this season, so maybe she helps lead the girls to the diner.