Kahler-Jex was a doctor of the Kahler race who provided scientific advancements to his people to aid them in the war raging on their homeworld. Like others of his species, he had a unique marking on his face.
Kahler-Jex performed hideous experiments on members of his race in order to transform them into cyborg war machines. One of them, Kahler-Tek, later known as "the Gunslinger", failed to be shut down once the war on his planet ended. The Gunslinger hunted down the people that experimented on him and Jex escaped to Mercy in the United States. As self-inflicted punishment, he decided to aid the people with electricity from his ship and cured the townfolks' cholera. He became good friends with the marshal.
Tek tracked Jex to Mercy and cut off supplies to the town. Several weeks later, the Eleventh Doctor, Amy and Rory arrived. Finding Jex' ship, the Doctor discovered his crimes. He nearly handed Jex over to Tek, only to be talked down by Amy. Mercy's marshal Isaac was accidentally killed when he pushed Jex out of the path of Tek's weapon. Tek made an ultimatum: hand Jex over by noon the next day or the town would be destroyed. In a duel, the Doctor distracted Tek and Jex escaped to his ship. Jex - feeling guilt for the experiments he conducted, and knowing that if he fled to another world then Tek would just follow him and another world would be caught up in Tek's quest for revenge - committed suicide by blowing up his ship. (TV: A Town Called Mercy)
Kahler-Jex was a remorseful and - inwardly - selfless person, devoted loyally to ending the war on his home planet. Kahler-Jex committed atrocities by experimenting on members of his own kind, but he only did so because he wanted to bring peace. He was religious which was shown by his belief that all the people he had wronged would be waiting for him in the afterlife and that he would have to carry their souls up a mountain. Just before he died, he stated that he hoped the souls would be kind. Despite what he had done in the past, Jex regretted his crimes for the rest of his life. He claimed that could hear the screaming of the victims of his experiments every time that he closed his eyes. It was because of his shame, and his unwillingness to face Kahler-Tek, who believed he was a coward, that he eventually committed suicide. (TV: A Town Called Mercy)