Kahler-Jex is the secondary antagonist of the Doctor Who episode "A Town Called Mercy". He is a doctor of the Kahler race, and the creator of the Gunslinger, formerly known as Kahler-Tek.
On his homeworld of Kahler, Kahler-Jex performed scientific experiments on members of his race, turning them in to cyborg war machines, who eliminated any who were deemed an enemy of the state. When the war on the planet ended, all of the cyborgs were shutdown, however one was accidentally not. The last cyborg, Kahler-Tek, hunted down the scientists. Two of them escaped to Earth, only one of them actually surviving to make it to a western town, Mercy. He provided the town with electricity and cured their cholera, in the process befriending the town marshal, Isaac.
Kahler-Tek, or "the Gunslinger", as he was known, found where Kahler-Jex was hiding, and cut off all supplies to the town. Weeks later, the Doctor and his companions appeared. After almost handing him over to the Gunslinger for his crimes, accidentally having Isaac killed in the process, they formed a plan. The Gunslinger returned at noon the following day. They distracted Kahler-Tek, allowing Kahler-Jex to return to his spaceship. Knowing that he had caused too many problems for all, Kahler-Jex made his ship self-destruct, finally completing the Gunslinger's quest for revenge.