Who is Marie de Medici/The Dowager Queen of France from Confessions of an Ugly Stepsister and what is their importance?

Marie de Medici is an old woman who fears for her throne. She is obese and knows her life is almost over, so she wants to commission an artist to depict her. She plans a ball to find a non-royal wife for her nephew, Philippe de Marsillac. Margarethe respects Marie as she manipulates the people around her to get what she wants. The queen proves finicky at the ball and is the person who discovers the fire in the Pruyn household.