Who is Mademoiselle Marie la Vicomtesse de Graay from Lord Hornblower and what is their importance?

Mademoiselle Marie la Vicomtesse de Graçay is always referred to simply as Marie, She is the widowed daughter-in-law of the Comte, and she derives her title entirely through her marriage to his deceased son. Marie grew up a peasant, and she met the Comte's son while he was billeted in her town during military maneuvers. Shortly after their marriage, her husband was killed in action.

Marie functions as a daughter to the Comte, and she occasionally refers to him publicly as her father. Marie is described as pretty, strong, and fair haired. Hornblower finds Marie to be beautiful and attractive, but he realizes that many men would not consider her to be beautiful.