Kahler-Tek was originally from Gabrean on Kahler. He was one of many who were experimented on by Kahler-Jex and his team to create a soldiers that would end the war their people were engaged in. Tek was transformed into a cybernetic war machine along with countless others who ended the war within a week. (WC: The Making of the Gunslinger) However, during that time Kahler-Tek was damaged in the field leading to the resurgence of his original personality. This meant he did not obey his pre-programmed instructions to deactivate after the war ended and began hunting down the scientists who experimented on him.
He tracked his final target, Kahler-Jex, to the town of Mercy where he had become the town doctor. He then placed the town under siege, as if he went in, innocent people may have gotten in the way of his gunfire, so he waited, suspecting they would throw him out. However they didn't.
Three weeks later, the Eleventh Doctor arrived with his in-laws, Amy and Rory and learned about the Gunslinger's presence in the area. After learning what Kahler-Jex had done to him, the furious Doctor threw him out of the town but allowed him to return before the Gunslinger could kill him. Tek accidentally killed Jex's friend, Isaac, who sacrificed himself to save Jex from the Gunslinger's energy beam. Frustrated, Tek warned that he would return the next day and kill everyone unless the Doctor handed Jex over.
Tek returned, as he had promised but did not go on a murderous rampage, sparing everyone in the town despite the threat he'd made the day before. When he realised that Jex had left he was enraged but Jex contacted him from his ship, expressing regret about what he had done to him. Tek demanded that Jex return and face him but instead Jex chose to commit suicide.
Believing that he no longer had a purpose, the Gunslinger decided to go into the desert and self-destruct but the Doctor persuaded him to stay and act as the town's protector. (TV: A Town Called Mercy)
Kahler-Tek was strongly motivated by vengeance. He was determined to kill the entire team of scientists that had turned him into a cyborg. However, he demonstrated an unwillingness to harm innocent people. Although he threatened to kill everyone in the town of Mercy, he did not intentionally harm any of the people that lived there, even though they had not handed Kahler-Jex over to him. After Jex committed suicide, the Gunslinger felt that he no longer had any reason to live and decided to go into the desert and self-destruct. The Doctor convinced him to stay and act as a protector to the town. (TV: A Town Called Mercy)
Built to be a weapon, Kahler-Tek was armed with a powerful energy weapon. The weapon was linked to an automatic targeting computer, capable of calculating the probability of harming innocents, advising him based on this to either disengage or terminate the target. He also had sensors that could see through walls and rocks, and identification software that could seemingly detect heartbeats and vital signs and identify Kahler markings. He also possessed a short-range teleport. (TV: A Town Called Mercy)
Other Information edit
When cyborg hacker Psi was attempting to protect Clara Oswald from the Teller, he ensured that it would focus on his guilt by uploading the memories of some of the worst criminals in history, one of them being Kahler-Tek. (TV: Time Heist)