Who is Joan Harris from Short Story Masterpieces and what is their importance?

Joan Harris is a friend of Jack Lorey. She is described as a woman of questionable ethics and morals, who indulges in a series of relationships with men who are both questionable and dangerous. In the end, however, we learn that Joan chooses her lovers not for their abusive behaviors towards her, but rather for their abusive behaviors towards themselves. Joan is seen to be a "black widow" of sorts.