Candace, it's Ralph. Tell dad there's no way in hell I'm spending nuclear Armageddon trapped with him in a fucking closet. You can have my number. It's "6." Now if you'll excuse me, I've got a mushroom cloud to walk into. Have a happy holocaust!”— Ralph, on his holotape
Ralph Keller is part of the Keller family. He does not actually appear in the game, but is mentioned in the Keller family transcript.
As Ralph mentions in a note to his sister, he had no intention of taking shelter from the imminent disaster and being restricted to a confined location with his father, deciding instead that he has "a mushroom cloud to walk into." This indicates that he commited suicide via a nuclear explosion.
Ralph Keller is mentioned only in Fallout 3.