Who is Richard "Ricky" M. Knapp from Under the Banner of Heaven: A Story of Violent Faith and what is their importance?

Richard (Ricky) Knapp is a drifter who is traveling with Ron and Dan the summer they murder Brenda and Erica. He is sitting in the car outside Allen and Brenda's apartment when Ron and Dan commit the murders. Dan and Knapp meet while working together on a construction site in Wichita, Kansas. Knapp is fresh out of jail and has no place to stay so Dan offers to share the Impala with him. Knapp takes Knapp leaves with Carnes after the murders, abandoning the Lafferty brothers at the motel.