Justin was a Glader who's name was seen engraved on the wall of the Glade in The Maze Runner film. His name was crossed out at the time, therefore he was dead when Thomas arrived. It is unknown what he looks like and is never mentioned.
In Maze Runner: the Scorch Trials the name Justin is called for testing. it is highly unlikely that Janson was referring to this character.
In The Scorch Trials Official Graphic Novel Prelude, Justin is said to have been Minho's running partner and to have fallen to his death while running.