Meredith Sorenson is a recurring antagonist in "Pretty Little Liars".
For most of the series Meredith was mostly an antagonist to Aria but she eventually grew into a full-on villain in the third season, when she reveals a mental side.
She was portrayed by Amanda Schull.
Aria seems to have a rivalry with Meredith, as she remembers when she caught her father kissing Meredith in the back of a car. Meredith continues re-seeking Byron's affections, much to Aria's annoyance.
In Season 3, Aria's English teacher goes on maternity leave and is replaced by Meredith, much to the chagrin of the Liars. Aria is texting about Meredith, and she holds the liars after class to talk. Meredith lies to Byron about how class went, saying Aria was "cautiously welcoming."
After Meredith's class, someone vandalizes Mona's locker, and the liars worry people will suspect them, but Spencer tells them that they were being held after class by Meredith. Emily shows a lack of faith in using Meredith as an alibi.
During the episode Mona-Mania, Meredith is harmed in an explosion, (which was later revealed to be the work of Mona Vanderwaal) she assumes Aria and her friends were responsible, which harms Byron and Aria's relationship.
After Aria hears Byron arguing with Meredith, she follows Meredith to a cafe and learns from her that Byron really wasn't with her mother the entire night that Ali went missing. She also knows that Ali was blackmailing Byron, which would've given him motive and opportunity to kill her.
Meredith learns that Aria has the pages from Ali's diary, which confesses to blackmailing him. The next day, Aria comes down with the flu, and Meredith gives her herbal tea, which will supposedly make he feel better. Through a "dream" in which Ali visits her, Aria learns that Meredith drugged the tea to give her an opportunity to find the diary pages, to protect Byron.
Aria learns that Meredith locked her in her room and unsuccessfully attempts to escape. When Meredith returns to the room to confront Aria about where she hid the pages, Aria escapes the room and Meredith hits her in the head with a blunt object, which knocks her unconscious, and puts her in the basement.
When Hanna and Emily go to Aria's house, as they witnessed Meredith buying insomnia medication, and remember that she was "nursing" Aria back to health, Meredith locks them in the basement with Aria. When Byron arrives home, Meredith tells him that Aria and her friends know everything and have proof implicating him in the murder. He rescues them from the basement and Meredith disappears.
Later, Byron says when the police arrived to Meredith has left the city. She apparently had a history of mental illness and was not taking her medications.