Who is Lisa from Black, White, and Jewish: Autobiography of a Shifting Self and what is their importance?

Lisa is one of Walker's friends in California. She is white, beautiful, and Walker admits to being jealous of what she perceives as Lisa's privileged life as a model. Walker later learns that her jealousy was unfounded, and that Lisa's "photo shoots" were actually sexual sessions, in which Lisa was molested. Walker accompanies Lisa to the police station where she reports the attacks.