Peter was a Glader whose name was seen engraved on the wall of the Glade in The Maze Runner film. His name was not crossed out at the time, therefore he was alive when Thomas arrived. He is never mentioned by any of the Gladers. Peter is presumed to have died during the Griever attacks, since he is never seen or mentioned afterwards.
Peter name

Peter's name as seen engraved on the wall

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Peter is seen crouching down under Winston.