Peg Muldoon is a Goblin and the sister of the brothers Muldoon.
It is likely that she was turned into a troll by Skulduggery Pleasant at the same time he turned the Muldoon's into goblins. It is even briefly mentioned that Skulduggery threw an entire mountain at her in an attempt on her life, but she survived.
Despite their history, Peg fell in love with Skulduggery, dressing up as a bride by sewing together dozens of wedding dresses. When Valkyrie Cain payed the Muldoon's ransom for babies, Peg threw Skulduggery through the church roof. Skulduggery engaged Peg in combat, throwing a church pew and rubble at her to no avail. It ended with Skulduggery talking to her, saying that they were not destined to be together. A tearful Peg left the church, in search of another man to fall in love with.