Mona Vanderwaal. Mona is a former unpopular girl at Rosewood who longed to be
accepted into Alison DiLaurentis's group and was continuously mocked by Alison.
After Alison disappeared she befriended Hanna Marin and they together, became the
‘It Girls’ of Rosewood High. At the end of Season 2, she is revealed to be ‘A’;
the person who has been terrorizing the 4 Liars. Mona is being controlled by a
person in a Red Coat. She was sent Radley Sanitarium for psychological and
emotional treatment. It is known that she frequently escaped from Radley to
inform other A-Team members what to do. Soon she sides with the Liars and slowly
starts becoming one of the Liars and starts being terrorized by ‘A’.
Lucas Gottesman. Having been frequently tormented by Alison, he often
expressed his happiness at her death, going so far as to destroy Alison's
memorial; that the Liars built, believing her cruelty shouldn't have been
forgiven posthumously. Soon afterward, due to a series of misunderstandings and
coincidences, the Liars suspect Lucas may be A; it is later revealed that Lucas
had gambled away thousands of dollars of Caleb's savings. Following this, his
friendship with Hanna and Caleb is severely strained, and Lucas grew
increasingly more troubled. It was revealed in the third season that he had
been visiting Mona at Radley, because she had discovered he was selling test
answers and was blackmailing him into cooperating with her. He moved away at
the end of season 3, out of fear and as a way to start over.
Toby Cavanaugh. Toby is a shy person and Jenna's older
step-brother. The Liars believed Toby used to spy on them from his tree house
into Emily's bedroom window, watching them change clothes. Alison and her
friends tried to get revenge on him by throwing a stink-bomb into the Cavanaugh's
garage, accidentally, though it is debatable whether or not Alison was after
Toby or Jenna; blinding Toby's stepsister, Jenna Marshall, in the process.
Alison blackmailed Toby into taking the blame for Jenna's "accident",
by saying that she would tell the whole town about one of Toby and Jenna's darkest secret.
Spencer Hastings. The ‘smart girl’ of the group. She is an
extreme perfectionist who always tries to please her wealthy parents who
clearly show more affection towards her older sister, Melissa. Spencer had been
the only person brave enough to stand up to Alison, and may have been one of
the last people to see her alive. Spencer then joins the "A" Team to
find out more about Red Coat, the mastermind behind hurting her and her
friends, and to find out the truth about her boyfriend, Toby Cavanaugh; who
turns out to be doing the same thing as her, getting more information on Red
Coat.
CeCe Drake. The first time The Liars see CeCe Drake, they think she
looks and acts a lot like Alison. She was very close to Alison before she died
a fact unknown by her other friends. CeCe dated Alison's older brother Jason.
As a close older friend of Alison, she would take her to parties and mentor
her. CeCe knew all about the four girls and their drama, while they knew
nothing of her. Jessica DiLaurentis; Alison’s mother reveals that CeCe and
Alison's friendship was obsessive and toxic, and that they imitated each
other’s personalities.