Alison is revealed to be "A" and a psychopathic serial killer who murdered a total of seven people.
Alison DiLaurentis was once a cruel bully and an ultimate liar who always wanted to get what she wanted most. She would treat Aria, Emily, Hanna and Spencer specially, although she would manipulate them and bully them as well. Alison was known as the "Queen Bee" of Rosewood and a popular girl, making many jealous.
In secret, "Alison" was actually Courtney DiLaurentis, the twin sister of the real Alison. One night, the real Alison, whom Courtney had tricked their family into believing was her, kills Courtney with help from Nick Maxwell. Courtney is identified and Alison and a hunt for her killer begins.
- In the television series, Alison is much less villainous and redeems herself over the course of the series. She is also not "A". The characteristics of Alison from the books are given to Charlotte DiLaurentis and Alex Drake.