"-A"

"A" is the main antagonist in the series. From the opener, "A" sends messages to Aria, Emily, Hanna and Spencer taunting them about knowing all their secrets and plans to make their lives miserable, as Alison and they had made everyone else's lives miserable. At first, the girls suspected Alison as "A", but her corpse was unearthed in her backyard. The most possible suspect is Jenna Marshall, who was a victim of Alison's prank. With Jenna's vindictive tendencies, she could be "A". During the mid-season one finale, while Ezra and Aria were in the car, Noel Kahn wrote "I see you" on the back of Ezra's car, prompting Hanna to believe Noel is "A". The girls continually think Noel was "A", possibly but "A" also went to great lengths to destroy any evidence, like cutting down a tree that had "Alison + Ian" carved on it. However, "A" has also done good things for the Liars, such as saving Ezra and Aria's relationship by framing Noel for cheating on midterms. Although "A" did some good, "A" decides to blackmail Hanna into doing things she doesn't want to do, like torture Lucas and almost ruin Aria and Ezra's relationship. In return, "A" gives her money, to help combat the financial difficulty that her and her mother are going through. In the season one finale "For Whom the Bell Tolls", "A" saves Spencer from dying by pushing Ian off the ledge near the bell in the church while he was trying to push her to her death, prompting Ian to be hung by the ropes of the bell. When the police come, however, Ian's body is nowhere to be found, and neither is "A". "A" has taunted Aria about her relationship with Ezra by sending Aria a text about their relationship which led Ezra to break up with her and the fact that she knew about Byron's affair with Meredith, Hanna about the fact that she shoplifts, and was bulimic, Spencer about her flings with Ian and Wren, and Emily about her sexuality, and faking a college acceptance letter. While the girls went back to the graveyard at night, most likely A turned the video of Ian supposedly strangling Alison on with the real ending of Alison getting back up saying "Thank you for coming tonight" to Ian and her walking away. After discovering that Alison was ambushed and buried alive, the girls now think that whoever is "A" must be the real murderer of Alison and framed Ian for Alison's murder. In "Touched by an 'A'-ngel," "A" discreetly gives Emily massages when she arrives at a spa appointment to shake her up. Later "A" is seen talking to the Liars therapist. In the episode, "I Must Confess," the girls realize that they have made a crucial error by telling Dr. Sullivan about "A". After "A" sends Dr. Sullivan a message saying, "And I'm the first person you've told about this?", on a loop, Dr. Sullivan then decides to do a little bit of investigating herself, realizing that she knows who "A" is. This then leads to "A" kidnapping her and "A" taking her key to the building, and taking the files about "A's" identity and sessions with Dr. Sullivan, leaving the girls with no evidence about who "A" is. The story is continued in episode "Over My Dead Body."