How is Mona A from Pretty Little Liars?

Answer 1

in the 4th book mona is a because she finds alisons diary in a pile of alisons old stuff that was getting thrown out because Maya was moving in. and since the girls and alison where so mean to her when she was dorky, she wanted revenge.

Answer 2

In the beginning of the book there is a box outside of alis old stuff. In there was alisons diary... Mona took it and read all their secrets for revenge.