Gray (Fragments)

Gray was the younger brother of Jack Harkness, and was a small boy when Jack lost him. While the Boeshane Peninsula was being invaded by an unknown alien race described as "the worst possible creatures you could imagine", Jack was told to run away with Gray to safety. But Jack accidentally let go of Gray's hand and ran back. Their father Franklin was killed in this same attack, leaving their mother a widow. Jack never saw Gray again, or so he thought. (TW: Adam)

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

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

It was revealed that when Gray was taken by the creatures that attacked the Boeshane Peninsula, he was tortured for several years. Corpses building up around him, he wished that he could join them. Soon everyone was dead and the creatures had left, leaving Gray with the "terrible things" he had learned at their hands. Captain John Hart managed to have saved him but soon realised Gray had changed. Gray let his hatred of Jack for letting go of his hand consume him, becoming psychotic. He molecularly bonded a bomb to John's wrist and threatened to detonate it if John didn't cooperate. He made John take Jack back to 27 AD Cardiff where John buried Jack to live forever buried beneath Cardiff. He then set about destroying everything Jack cared about.

Gray came to the 21st century and made all the Weevils come out of hiding. He then shot Toshiko Sato which ultimately lead to her death. A banging started from the Vaults which was revealed to be Jack stored in Cryogenics, as he was unearthed in 1901. He told Gray that he'd forgiven him, but Gray refused to listen, so Jack tearfully chloroformed Gray and put him into storage until Jack thinks he would emotionally recover from the horrifying events, which may be never. (TW: Exit Wounds)