Kahler-Jex is a supporting character from the Doctor Who episode "A Town Called Mercy". A member of the Kahler race and a brilliant scientist, Jex fled his homeworld when one of his own creations sought to kill him. He was portrayed by Adrian Scarborough.
At some point in Earth's 19th century, the Kahler homeworld was engulfed by war. Nine years into the war, the planet had been decimated with a large percentage of the population wiped out. In order to bring the war to an end, Kahler-Jex and several other scientists conducted a series of horrific experiments on military volunteers, turning them into powerful cyborg assassins. Many of the volunteers died during the modification procedure, but those that survived became the ultimate killers and soon brought the war to a close after slaughtering most of the opposition forces.
With the Kahler war over, the cyborgs were decommissioned. However, one of the cyborgs - Kahler-Tek - failed to shut down and started hunting the scientists that experimented on him. Jex escaped from his homeworld in a small craft and went into hiding, eventually finding his way to Earth where he settled town in the small town of Mercy in the western United States.
During his self-imposed exile, Jex provided his scientific expertise to Mercy's community, building a primitive electrical grid for the town using his ship as a generator. He also used his medical knowledge to cure an outbreak of cholera in the town and was regarded by the townsfolk as a hero, particularly by the town marshal Isaac. However, no one knew of the crimes he had committed back on his home planet, though Jex's past would come back to haunt him as Kahler-Tek tracked him down. Tek, whom the people of Mercy would refer to as "The Gunslinger", embargoed the town and would not allow any food, medicine or reinforcements across the town border until the people gave up Jex.
Three weeks after Mercy was cut off from the outside world, the Doctor arrived with his companions Amy Pond and Rory Williams. They encountered the Gunslinger, but the cyborg did not attack as they had stepped over the town line and the Gunslinger refused to attack any innocents. After meeting with Jex and Isaac, the Doctor offered to help fix the town's electrical line and went out into the desert where Jex had left his ship. The Doctor accessed the ship's computer in order to find out more about the Gunslinger and discovered Jex's past crimes. Disgusted, the Doctor almost handed Jex over to the cyborg, but the townsfolk interfered. When the Gunslinger appeared to take his revenge, Isaac shoved Jex aside just as the Gunslinger fired his weapon, accidentally shooting the marshal. The Gunslinger then warned everyone that he would return for Jex the next day at noon and that he would kill everyone who interfered.
Jex feared for his life and the townsfolk were also frightened, banding together to drag Jex out of town. The Doctor, having been appointed as the new marshal, was able to defuse the situation and protect Jex.
When the Gunslinger returned to Mercy, Jex secretly fled the town while the Doctor and the townsfolk tricked the cyborg into pursuing them all over town by disguising themselves as Jex. The plan was to buy enough time for Jex to take his ship and escape to another planet, but Jex knew that the Gunslinger would never stop pursuing him and would threaten the inhabitants of whatever world he took refuge on. Jex decided to face his fate and activated his ship's self-destruct system, killing himself.