Who is Diana Villiers from The Fortune of War and what is their importance?

Diana is well known for her beauty and sexual allure, captivating and holding the attention of most every man she meets, including Maturin. She is Jack Aubrey's cousin by marriage, and a native of England. Leaving England and emigrating to America, where she became a citizen was a direct result of trouble at home, and a desire to avoid complications after Louisa Wogan was arrested. When Maturin arrives in America, Diana sees him as a source of rescue.... her knight in shining armor.