Sister Peg was a Catholic nun who lived and worked in New York City. Most of her work involves helping and protecting prostitutes. As such, she sometimes comes into contact with Manhattan SVU Detectives Elliot Stabler and Olivia Benson.
Sister Peg is abducted by the self-proclaimed psychic Sebastian Ballentine and his wife Carlene. She is rescued, and reveals that while she was captured, she learned that the Ballentine's baby was one that was kidnapped from someone in Canada. Thanks to her information, the police are able to track down the baby's father and give him back his son. (SVU: "Pure")
During an investigation into John Victor Bodine, Sister Peg agreed to take in Belinda Holt, an underage prostitute. When Bodine was able to escape from prison, he tracked Belinda down and kidnapped her, assaulting Sister Peg in the process. She was hospitalized and survived. (SVU: "Underbelly")
Sister Peg then began assisting the homeless community. She helps Olivia and Elliot catch a killer who turns out to be client of her shelter, only to be accidentally shot in a shooting spree carried out by the victim's daughter. Olivia tries to save her, but she ultimately dies. Her shooter, Jenna Fox, is killed by Stabler when he is forced to shoot her to stop her spree. (SVU: "Smoked")