A (Book Character)

"A" in the book series is an anonymous character who sends threatening texts and acts as a living nightmare to the girls, revealing secrets that they thought no one knew, or only Alison knew.

Each of the Pretty Little Liars receive texts and don't know that the others are getting them as well, as they keep them to themselves in fear of their secrets being revealed. Each of them believe that Alison is somehow alive or dead, sending the texts from beyond her grave for revenge. Only when Alison's body is discovered do they open up to each other about the texts and realize that the sender isn't Alison.

They found out in "Unbelievable" it was Hanna's friend Mona. Mona wanted revenge for the ridicule she suffered when she was a loser. She knew all of the Pretty Little Liarss' secrets that only Alison knew because when Maya moved in, Alison's stuff was put in a box on the sidewalk which, unknown to everyone, held Alison's (which turned out later to be Courtney's) diary and Mona found it. Later Mona is killed because she wants Spencer to join her and be "A" but Spencer refuses so Mona tries to push her off a cliff but ends up dying herself. After Mona dies, the real Alison becomes "A".

In "Wanted" everything is revealed in a letter she gives to the liars when she traps them in a house and sets it on fire.

The Alison that they had been friends with had been Courtney all along. Courtney was Alison's slightly mental twin sister. Courtney always said she was Alison and acted like she was. One day she took Ali's A-is-for-Ali ring and pretended she was Alison in front of the Pretty Little Liars. Since she didn't have the ring, Alison's mother thought she was Courtney and Alison got Courtney's life and Courtney got Alison's.