Television Series
Mona Elise Vanderwaal is a German-American student at Rosewood High School. She is a former nerd that often got picked on a lot by Alison but still wanted to be part of Alison DiLaurentis' crowd, instead she got insulted and humiliated by them. After Alison's disappearance Mona transformed herself and became the Queen Bee with Hanna Marin. Mona is obsessed with popularity and is terrified the strange things Hanna does will make them unpopular again. She's a shoplifter, but has never been caught. She and Hanna became best friends when Alison's crowd got disbanded until the episode Keep Your Friends Close when Mona heard a rumor that Hanna got skinny using liposection, and said they weren't friends anymore. Mona visited Hanna in the hospital and apologized, and they made up and became BFFs again. She also said that the reason she wanted to be Hanna's friend was when she saw Hanna bow after puking on the trampoline, and Mona thought that was a major life lesson. She threw a surprise "Welcome Back!" party for Hanna, but Hanna wasn't in the party mood.
Book Series
In the books, Hanna is able to figure that Mona is A and she rushes to tell the other girls. Mona runs her over with a car putting Hanna into a coma so she will forget. Mona pretends as if everything is normal with Hanna to keep up appearances. Mona sends herself a threatening text message to throw them off her trail. She tricks Spencer into coming with her by telling her that they are going to the police. The girls realize Spencer is in danger because she is trapped with Mona after figuring out Mona is A and they call the police. Mona reveals to Spencer she planned this all along to get revenge on the girls for constantly teasing her.
Mona Vandervaal
She says that she found their secrets from a diary Courtney pretending to be Alison. She then asked Spencer if she would be A with her because Mona had grown to like her even though she had planned to frame Spencer for killing Alison at first. Spencer rejected her offer which leads to Mona attempting to kill her. Spencer is able to avoid Mona which causes her to fall to her death. She is played by Janel Parrish.