Bill Stevens was a government employee working at federal surveillance center K-21B before the Great War. When the bombs fell, Stevens had been promised a place in the bunker beneath his workplace in the event of total atomic annihilation.
He was rushing to get something from his car as the bombs were falling and was therefore left outside by his co-workers who couldn't risk the bomb destroying the bunker. He was killed by the nuke, and his skeletal remains can be found beside a suitcase outside the bunker in the abandoned shack.
Bill Stevens' remains appear only in Fallout 4.