Dave Mather is a legendary gunslinger who was killed in the 1800's and later eaten by an unnamed ghoul who stole his identity.
The only things that are known about Dave Mather is that he was a famous gunslinger and at some point he was killed and later eaten by an unnamed ghoul, who stole his identity by using his face, transformed into him and got his memories in order to look like the real Dave Mather.