Maggie was born in 2268. Billy Creel took in Maggie at the age of 3 and brought her to Megaton. Maggie remembers nothing of her life before Megaton. A Megaton resident caught a bullet in a raider attack and Billy was given the man's house. He's been in Megaton ever since, raising Maggie as well as you can expect from a carefree wasteland adventurer, even telling her the combination to his lost safe. Of course, there are a few around town who say that Billy was the one behind that bullet that bought him his place in Megaton.
Her real parents died during a raider attack in 2275. She's a friendly little girl running around the town. She appears to be interested in Harden Simms and discusses with Billy the prospect of her kissing the boy, to which Creel tells her to slow down, as she has plenty of time to do that later. She also asks Billy Creel about "The Talk", stating that Harden has already had the talk.
Maggie goes to live with Manya and Nathan in the event of Billy's death.
On Colin Moriarty's terminal, Colin states that he believes Maggie wasn't rescued and that it was probably Creel himself who killed her parents. However, nothing else in the game supports this theory.
If you steal Holodisk: The Guardians of Gillyfrond from Billy Creel's house, regardless of whether you are caught or not, Maggie will tell you she doesn't like you because you took things that don't belong to you.