Stephen was a glader who was around at the time of George's death, he was the person who stepped up and when down with the box, but it killed him off-screen, confirming his fate.
Stephen was first mentioned by Thomas in The Maze Runner. When Thomas went to the Deadheads, the first grave he saw was Stephen's. His name on the tomb had the "n" engraved extra small at the edge of the stone, showing that the Bagger in charge of the burial had underestimated the space needed for his name. Thomas felt an unexpected but detached sorrow for him at this. He most likely died in the maze.
Stephen could have been named after Stephen Hawking, an astrophysicist.