Isaac (A Town Called Mercy)

Isaac was the first of three United States Marshals based in Mercy, Nevada in 1870. He protected Kahler-Jex because the alien had helped to rebuild their town with electricity and stopped a cholera outbreak. He referred to Jex as "Doc", and both men saw each other as a friend. He was also the last human marshal for Mercy for some time, giving the title to the Doctor, who then gave it to the Gunslinger.

Biography
He first met the Eleventh Doctor when he saved the Time Lord from being caught by Kahler-Tek — a cyborg more commonly known as "the Gunslinger". Since Isaac had already reasoned that the cyborg was in fact looking for Kahler-Jex, he knew the town was wrong to try to offer the Doctor to the gunslinger (who demanded "the doctor"). He and Rory distracted the Gunslinger in the desert while the Doctor went out to find Kahler's space ship.

Upon realising that the Gunslinger was going to kill Jex as a war criminal, he pushed Jex out of the way and was killed himself. As he lay dying, he made the Doctor the new marshal and told the Doctor to protect Jex from harm.

His death weighed heavily on Jex's conscience because of their friendship; Jex believed Issac would be one of the souls he wronged and would be carrying in the next life. (TV:  A Town Called Mercy)