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

Dick is a frequent member of the James Gang activities but falls out with Wood Hite, who is Jesse's cousin, after Dick has an affair with Wood's stepmother. Dick turns himself in to the police in the hope that he'll be able to help capture Jesse James and claim at least a portion of the reward for himself. He is the only person to testify against Frank James.