Paige was a Medic Program and she saw some programs being harassed, she ran to go save them. The Programs were named Quorra and Ada, while she was helping them, she learned they were ISOs and was about to report them, but Quorra convince her not to. However, the Hospital was attack and Ada knock her out, when she woke up, she was confronted by Tesler who was searching ISOs, she then learned her friends including Roxy were derezzed. Tesler told her that ISOs killed her friends and Paige believed him and joined him and his forces. Unaware to Paige, Tesler order the execution of her friends.
A field commander with unmatched combat skills, Paige has endured her share of tragedy. Separated from her Grid "family" during a battle, she was rescued by Tesler. This resulted in a deep loyalty to him, which occasionally blinds her to his darker side. She is Beck's chief antagonist and eventually, the object of his desire.
Paige is blinded by her loyalty to Tesler and ultimately believes she is fighting for the right side when in reality she is helping to oppress the programs of the Grid.
For this innate goodness, Tesler himself does not give Paige his full trust, as he killed a program Paige had promised would be pardoned without her knowing.