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 round 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 had plenty of time to do that later.

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 without any explanation.

Maggie also apparently goes to live with Manya and Nathan after Billy's death. The cause of his death and if this event is triggered by some other event is unknown.

In one of the NPC conversations between Billy Creel and Maggie, she admits to liking Harden Simms and asks how to go about giving him a kiss.

Appearances
Maggie only appears in Fallout 3.