Who is Ruth Woodmoore from Taft and what is their importance?

Ruth Woodmoore is Marion's sister. When she was a teenager, she covertly kissed John Nickel while he was dating her sister. One night, John goes to the Woodmoore house for dinner and discovers that Ruth has returned to Memphis after falling on hard times in Detroit. Ruth begins to frequent Muddy's Bar, and one night she and John have sex. She becomes enraged when John says it was a mistake. She insinuates that she might tell Marion when she returns to town. At the end of the novel, Ruth helps John to Marion after he has been shot.