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

An outlaw during the 1800s, Jesse is cold and calculating but can be kind and generous with his family and friends. He is lauded by some as a hero but kills several men in cold-blooded murder during robberies. Jesse learns that Ed Miller is conspiring with some others to capture him for the reward money and kills Ed, hiding the body. Jesse marries a cousin, Zerelda Mimms, whom he calls Zee. They have four children prior to his death, including twin sons who die as infants.