Gray (Adam)

Gray was the younger brother of Jack Harkness. He later lost his sanity following years of torture, becoming consumed by a hatred for his brother. It remains unknown what happened to him following the Hub's destruction, (TV: Children of Earth: Day One) but it is most likely he perished in the inferno.

Early life
Jack and Gray were boys in the Boeshane Peninsula when it was invaded by an unknown alien race described as "the worst possible creatures you could imagine". Jack was told to run away and take Gray to safety. However, Jack let go of Gray's hand and lost him. He ran back but could not find him, and Gray was captured by the creatures. (TV: Adam) Now a prisoner, Gray was tortured for years. Corpses piling up around him, Gray wished to die as well. Eventually, the other captives died and the creatures abandoned Gray, leaving him chained to a ruined city in the Bedlam Outlands with the "terrible things" he had learned at their hands. (TV: Exit Wounds)

Being found
John Hart, a Time Agent and Jack's former lover, discovered Gray chained in a ruined city in the Bedlam Outlands, surrounded by bodies, the creatures having long abandoned the place and Gray. Hart said he had no idea how long Gray had been alone before he was finally found. John soon realised Gray had changed. Gray never forgave his brother and his hatred consumed him. (TV: Exit Wounds) While visiting the year 2009, Hart had given Jack the information that "they found Gray". (TV: Kiss Kiss, Bang Bang)
 * Whether John Hart had come under Gray's control at this stage was unclear, but not unlikely.

Revenge against Jack
Gray molecularly bonded a bomb to John's wrist and threatened to detonate it if John did not cooperate. (TV: Exit Wounds) Hart set a trap for the Torchwood 3 team and then let them see Gray in hologram form, giving the impression that John was the villain. (TV: Fragments) Having placed bombs around Cardiff, John detonated them to keep the rest of the Torchwood team occupied. Gray made John take Jack back to 27 AD Cardiff. Gray and Jack were reunited, only for Gray to stab Jack, presumably having learned from John about his brother's immortality. John buried Jack to live forever beneath Cardiff. Gray then set about destroying everything Jack cared for, travelling to 2009 and sending adversaries to the city through time and space. He also set Weevils out into the streets of Cardiff, thanks to a high-pitched sound which came from a vortex manipulator. He shot Toshiko Sato fatally while she was attempting to talk Owen through shutting down a nuclear reactor. The resulting overload caused the living-dead Owen to watch as his body collapsed.

However, Jack was revealed to have been cryogenically preserved in 1901 after the Torchwood team of that era had discovered him. John Hart had betrayed Gray by burying with Jack a ring that emitted a tracking signal he hoped the Torchwood team could use to find Jack in the present. Jack told Gray that he'd forgiven him, but Gray refused to listen. Jack tearfully chloroformed Gray and put him into storage until Jack thought that his brother could emotionally recover from the horrifying events, if at all. (TV: Exit Wounds)

Personality
After being tortured for many years, Gray went insane and grew to hate Jack, blaming him for the agony he had been put through. Gray stabbed Jack when his older brother embraced him and took sadistic pleasure in the thought of having Jack endure a cycle of deaths and resurrections. In addition to being sadistic and vengeful, Gray was ruthless and showed no remorse for committing murder, shooting Toshiko Sato and fatally wounding her. (TV: Exit Wounds)