Maggie is a resident of Megaton in Fallout 3. When she was 3 years old her parents were killed by Raiders; she was supposedly rescued by Billy Creel who took her back to Megaton with him. She's now 12 years old and remembers nothing about her parents. Colin Moriarty believes that Billy was actually the one who killed Maggie's parents and kept her as a trophy.
If you have the Child at Heart perk you can convince Maggie to tell you that there's a safe in Billy's house and she tells you the code to open it.