Mona Vanderwaal Is a character in the first four Pretty Little Liar novels. She was used to be an unpopular nerd who was teased by Alison and her friends, longing to be in their group. After Alison went missing, Hanna and her became friends and reinvented themselves as the 'it' girls of Rosewood Day. She is later revealed to be the first 'A', in revenge for all the bullying and suffering the girls put her through and for blinding Jenna, (in The Jenna Thing, which Mona witnessed). She tries to kill Spencer, but falls down Floating Man's Quarry and is killed. She also appears in Ali's Pretty Little Lies when Alison teased her, and in flashbacks from the 11th novel Stunning .

At first a dorky, chunky middle-schooler with braces and frizzy hair, Mona, along with her new BFF Hanna Marin, reinvented herself over the summer. As a teenager, Mona reinvents herself and is one of the most popular girls of the school. She has white-blonde hair, light blue eyes, and a few freckles. She has a large burn scar on her stomach from "The Jenna Thing".

In the first book, Mona is Hanna's best friend who shoplifts together with her. Mona used to be a dorky girl, but together with Hanna she transformed into a beautiful 'it' girl during the summer three years ago. She is invited to Noel Kahn's start of the term party where she makes out with his older brother Eric. Whenever the school's mean girls, Naomi and Riley say something mean about Hanna, Mona is there for her. Mona and Hanna get into numerous fights and arguments about Hanna's old friends, Emily, Aria and Spencer, because Mona thinks that she's been spending more time with them than with her.

As Mona's birthday is coming up, she chooses Naomi, Riley, Hanna and some other girls to be her 'next-in-lines' so she goes to buy dresses with them. She chooses beautiful champagne-colored dresses, but gets in a fight with Hanna and doesn't let her be at her birthday party. The fight occurred because Hanna wanted to spell out 'Get ready to party with Mona!' on the sky, but wrote 'Get ready to fart with Mona!' Mona becomes angry and leaves. There is only one more letter in the sky - A. Later on, Hanna gets a package with the champagne-colored dress, and thinks it was Mona who sent it. Happily, she puts it on and goes to Mona's party, even though the dress it too small. As she comes to the party, Naomi and Riley (who are in emerald dresses) give her weird looks, and when Mona comes out wearing the champagne-colored dress, Hanna realizes that A was the one who sent her the dress. Crying, she falls down and her dress rips, following the laughter of the party attendants, including Mona. Hanna tells Mona that she knows she got a liposuction because A sent her a message with the content, but Mona says that she shouldn't believe everything she hears and calls her a liar. Later, Hanna gets a message from A, but sees that it's not the usual blocked number. Even though she got a new phone and didn't put in the phone-book yet, she recognizes the number and rushes to tell the girls, but gets hit by a car before she can do anything.

After Hanna's car accident, Mona is there for her in the hospital every day, crying. When Hanna wakes up Mona apologizes, making them friends again. Hanna can't remember anything that happened at Mona's party. including the 'who is A' part. Later, Mona gets in a fight with Lucas regarding a secret that she won't explain to Hanna. She gets a text message and tells Hanna that it's from a person named A - of course, Hanna has been getting the texts too and admits everything to Mona. Mona seems relieved and tells the other girls about it, too. As Spencer and Mona are on their way to the police station, Hanna remember who A is - and it's Mona! The girls send a text to Spencer who tries to hide it, but Mona sees it and explains everything - how she found 'Ali's' diary and found out all about her secret, and how she read that 'Ali's' going to give Ian an ultimatum - either her or Melissa, bringing Mona to tell Spencer that Ian killed 'Ali'. Mona tries to force Spencer to join her, but they get into a fight and Spencer pushes Mona to the Floating Man's Quarry, causing Mona to break her neck and die that instant. After that, Hanna is shaken up, but at least now she knows who A was and who Ali's killer is. Mona gets all over the news as now the whole Rosewood knows about A, and the Rosewood Stalker - who was also Mona.

It is unknown how Mona came to be A, and it is also unknown if she knew about the twins. In Ali's Pretty Little Lies, it is revealed Alison was with someone when she killed Courtney. This is person could be Mona as she was the first A.