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.