In Pretty Little Liars , the identity of "A" is a central mystery and antagonist role that has been taken on by multiple characters throughout the series:

• Mona Vanderwaal was the original "A" during seasons 1 and 2. She stalked and tormented the main characters as the first revealed "A"

• Charlotte DiLaurentis (formerly Charles DiLaurentis) , Alison's transgender sibling, became "Big A" from seasons 3 to 6A. She took over the "A" persona to seek revenge and control, revealing herself in the Dollhouse storyline

• Alex Drake , Spencer Hastings' identical twin, was introduced in season 7 as "A.D." or Uber A. She continued the "A" legacy by blackmailing and torturing the Liars to uncover Charlotte's murderer

Thus, the main characters who have been "A" are Mona Vanderwaal, Charlotte DiLaurentis, and Alex Drake, each playing the role at different points in the series