Isaac (A Town Called Mercy)

Isaac was the Sheriff of Mercy, who protected Kahler-Jex because he had helped to rebuild their town with electricity and prevented a rise in Cholera. He referred to Jex as Doc.

Biography
He first met the Eleventh Doctor when he saved him from being caught by Kahler-Tek. He actually knew who the cyborg was after. He and Rory distracted the Gunslinger in the dessert while the Doctor went out to find Kahler's space ship.

When realising that the Gunslinger was going to kill Jex as a war criminal, he pushed him out of the way and was killed himself. He told the Doctor, before he died, to protect Jex from harm. (DW:  A Town Called Mercy)