Sedge is Partridge's dead brother. Except he's not dead. He was just mutated into a super-soldier. Surprise!
Sedge is pretty much written off as a non-entity for most of the story, but his shocking appearance and disappearance at the end is nothing short of bone chilling. When Partridge recognizes his voice, all that we are told in the beginning of the story can basically be thrown out the window. If Sedge is alive, then nothing Ellery told his son can be assumed to be true.