Mona Vanderwaal is a main character in the series. She is portrayed by Janel Parrish.
Mona was revealed to be "A" in "UnmAsked and was sent to Radley Sanitarium for treatment, wheresomeone else took her game. She was suspected of being murdered by 'A' but it was revealed in Welcome to the Dollhouse, that she is alive and has been living in A's house ever since she died and pretending to be Alison. She was the main antagonist of the first two seasons.