Maggie

Maggie is a girl from Megaton whose parents died during a raider attack. She was rescued by Billy Creel, who's been taking care of her ever since. She's a friendly little girl running around 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.

If you hack Colin Moriarty's terminal, however, it is revealed that he believes that 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 possess the "Child At Heart" perk, Maggie will inform you of a safe in the floor of Billy Creel's house, and give you the combination: 15 16 23 42 (a reference to LOST).

As a child, Maggie cannot be killed with any weapon and cannot be targeted using VATS. If you plant the charge on the Megaton bomb in The Power of the Atom, leave, and return to the city before detonating the charge, Maggie and Harden Simms will both have vanished.

Maggie also apparently goes to live with Manya and Nathan in the event of Billy's death.

It is revealed Maggie is from somewhere up north. This could be any number of places North of Megaton, being Megaton is fairly southerly.

Appearances
Maggie only appears in Fallout 3.