Arthur Gannon is an Enclave officer tasked with tracking down the mysterious package known as HR-D4. He tracked this package through the Commonwealth until he discovered its location at the FMS Columbia. There, he and a few other soldiers attempted to uncover HR-D4, but he was captured and his fellow soldiers killed by Janus. He was put into a cage alongside the captured HR-D4, and he eventually died of his injuries. He can be found in the cell right next to the player at the start of the game.
Arthur was, most likely, the ancestor of Arcade Gannon.