Who is Frank James from The Assassination of Jesse James by the Coward Robert Ford and what is their importance?

Jesse's brother, Frank is a participant in many of the James Gang robberies. Though he is not as cold-blooded as Jesse, he is a willing participant. At one point, Frank tries to extract himself from the gang's activities but sees Jesse's easy wealth and returns to the illegal activities. Frank turns himself in after Jesse's death and is tried for several crimes but is not found guilty and does not serve time.