Billy is a Glader and Bagger. It is possible he may have been named after Billy the Kid.
Billy was ordered alongside Jackson to take Teresa to the Pit.
He was mentioned afterwards and along with 19 other Gladers has been confirmed to have survived the Trials. It is confirmed Billy had survived the Scorch Trials and made it to the Safe Haven.
Billy is seen among the surviving Gladers at the end of the Maze Runner film. He then disappears in the transition to the Scorch Trials film.