Who is Mr. Johnson from Cities of the Plain and what is their importance?

Mr. Johnson is Mac's Father in law. His daughter (Mac's wife) died prior to the start of the novel, and he still hasn't recovered from the loss. Johnson tends to wander around late at night without any clothes on, or in just his long johns. John Grady won't allow the others call him crazy, and he often sits with him, listening to the man's stories of how things used to be in the west.