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

A frequent member of the James Gang, Ed Miller is in collusion with others to turn Jesse in to the law but Jesse finds out about the plan. Ed is at home one evening when Jesse arrives and he's immediately on edge. He tells Jesse that he fears Jesse might have heard rumors that Ed and others have met and that Jesse might come to the wrong conclusions about those meetings. Ed assures Jesse that there's nothing to the rumors that he plans to turn Jesse in for the reward money, adding that he has some savings of his own and has no need for the money. Jesse doesn't believe him and kills Ed, leaving his body hidden so that it's not found for some time.