Isaac was the marshal of Mercy, a good man who saw the best in people, called America the land of second chances and felt his town was called Mercy for a good reason...
Isaac was a man whose life was changed by two alien doctors. The first was Kahler-Jex, the physician he rescued from a crashed vessel a mile outside Mercy. He helped the stranger and even when the Gunslinger appeared, he stood by him, resolute in his belief that if they gave up Jex, ‘…we’re handin’ the keys of the town over to chaos.’ Even when some of the townsfolk declared that the alien should be left to his fate, the marshal stuck to his guns.
When the Doctor arrived his life was once again thrown into chaos but his kindness, integrity and courage never wavered. Even when he discovered the truth about Jex he still viewed the alien as ‘the guy who saved the town from cholera, the guy who gave us heat and light’. And ultimately he gave his life defending his old friend, pushing him out of the Gunslinger’s line of fire and taking the shot that had been intended for him. Isaac’s last act was to persuade the Doctor to take his place as Mercy’s marshal, and his final words were to the Time Lord, reminding him that he and Jex were alike: ‘You’re both good men...’ he said. ‘You just forget it sometimes...’