Who is Donald Martin from The Ox-Bow Incident and what is their importance?

Donald Martin is a young rancher who has brought his family from California to Pike's Hole in order to begin a ranch of their own. Martin has bought cows from Drew, but failed to get a receipt and one of his men has possession of Kincaid's gun. These facts together cause the vigilante mob to decide that Martin and his men killed Kincaid and to hang them. However, it turns out that Martin is innocent of the crime, as are his men.