Gray (Adam)

Gray was the brother of Jack Harkness. While the Boeshane Peninsula was being invaded by an unknown alien race, Jack was told to run away with Gray. But Jack accidentally let go of Gray's hand and ran back. Jack never saw Gray again. (TW: Adam)

John Hart mentioned to have found Gray, which shocked Jack. (TW: Kiss Kiss, Bang Bang)

Gray is seen as a hologram. (TW: Fragments)