Who is Mademoiselle Marie la Vicomtesse de Graay from Flying Colours and what is their importance?

Marie is the Count's widowed, daughter-in-law. She lives with the count as his daughter, running the household, and assisting Hornblower, Bush, and Brown during the winter they spend in the castle. Marie takes good care of Bush while he recuperates from his would and becomes involved romantically with Hornblower for a short time.