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.
She is abducted by the self-proclaimed psychic Sebastian Ballentine and his wife. She's rescued, but 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 (the mother was murdered when the baby was kidnapped) and give him back his son. SVU: "Pure")
Sister Peg then began assisting the homeless. In the season 12 finale 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's forced to shoot her to stop her spree. (SVU: "Smoked")