"A" is the main antagonist of the Pretty Little Liars novels and the television series of the same name. They are an anonymous Big Bad that target the main characters, the Liars, for their own motive and will stop at nothing to get what they want.
While there have been multiple of them, the original and first main "A" was revealed in the second season finale to be Mona Vanderwaal, the Liars' ally. Her helper was Lucas Gottesman. When Mona is sent to Radley Sanitarium for treatment, she is approached by an unknown person who wants to form a partnership, and that person becomes the second main "A", known as Big "A".
Mona and Big "A" form the "A-Team", hiring Toby Cavanaugh, Spencer Hastings, Lucas Gottesman, Darren Wilden and Melissa Hastings, while Big "A" commands the team in the disguise of Red Coat. After Toby and Spencer leave the team, Big "A" steals the game from Mona and kicks her out of the team, where she becomes one of the Liars herself. Big "A" joins forces with an ally, who is The Black Widow and also a decoy Red Coat, and the "A-Team" disbands after Big "A" murders Wilden for betraying them and Melissa leaves. In the 6A finale, Big "A" is unmasked as CeCe Drake, real name Charlotte, with her ally being Sara Harvey. In Season 6B after the five-year jump, Charlotte is murdered and this spawns the birth of the final "A", known as Uber "A", who wants to find her killer and get revenge. They hire Jenna Marshall and others as their helpers. In Season 7, it was revealed that Charlotte had other helpers, who were Noel Kahn and Jenna Marshall.
Instead of going by "A", Uber "A" uses the alias "A.D.". Over the course of Season 7, "A.D." hires multiple people assisting them in their "endgame". They create a board game known as "Liar's Lament" and send it to the Liars, as a way of watching over them and tormenting them. Sydney Driscoll was one of their many helpers, and Sydney escorts Aria Montgomery onto the team after they blackmail her with a secret she has been keeping. After Mona is revealed as Charlotte's killer, "A.D." reveals themselves as Alex Drake, Charlotte's half-sister and the identical twin sister to Spencer Hastings, one of the Liars. Aside from wanting to avenge Charlotte's death, she wanted to take over Spencer's life and become her, prompting to hold Spencer captive. Wren Kingston was Alex's partner-in-crime and her lover, and she had sent him to murder Mona in revenge for her killing Charlotte, however, she instead joins the team once more after offering her services. Alex was later defeated and she was captured by Mona, who then won the game overall.