Wadsworth

 is your personal Mr. Handy butler in Megaton, which you get, together with a house, for disarming the nuclear bomb that lies in the middle of the town. He can give you instructions on how to spruce up your house, give you haircuts and tell you jokes. Wadsworth can also give you Purified Water, but only up to five at a time before needing to recreate his supply.

If he follows you up to your bedroom, he can get stuck in there for good.

Appearances
 appears only in Fallout 3.