The character known as the Duchess de Polignac in the anime Lady Oscar is based on the historical figure Gabrielle de Polastron, the duchesse de Polignac. In the story, she is introduced as a singer at the Palace of Versailles, though she does not reside there. Despite her angelic voice and beautiful appearance, which cause many nobles to remark that she seems like an angel, she is revealed to be a cold and ruthlessly ambitious individual.

The Duchess de Polignac is primarily motivated by a desire for wealth, power, and social advancement. She is a deeply materialistic and unscrupulous woman who views everyone around her, including her own family, as pawns to be used for her own benefit. Her primary role in the narrative is as a negative influence on Queen Marie Antoinette, leading the lonely and impressionable queen down a path of scandal and poor judgment. She encourages the queen to gamble in clandestine casinos and uses her position as the queen's favorite for personal gain, securing money, honors, and influential positions for her family. When the queen faces a difficult situation, such as rumors about producing an heir, the Duchess advises her to lie and announce a false pregnancy, later cruelly blaming the fabricated miscarriage on Lady Oscar.

The Duchess has several key relationships that drive the story. Her friendship with Marie Antoinette is one of pure manipulation, as she abandons the queen without remorse to save herself when the French Revolution begins. She harbors a strong rivalry with Oscar François de Jarjayes, envying the queen's regard for the captain of the Royal Guard and becoming determined to eliminate anyone who counters her influence. The Duchess is also the biological mother of the kind-hearted commoner Rosalie Lamorlière. She gave birth to Rosalie as a teenager out of wedlock, using the name Martine Gabrielle de Goulard, and abandoned the child to protect her reputation so she could secure a wealthy husband. Years later, after being recognized, she attempts to blackmail Rosalie into coming to live with her, intending to force the young woman to marry the same aging and lecherous Duke de Guiche whose advances had driven her legitimate daughter to madness and suicide. Her treatment of young Charlotte de Polignac, whom she plans to sacrifice in an arranged marriage to the wealthy Duke for the sake of family finances, leads directly to Charlotte losing her reason and killing herself.

While the Duchess appears to be an influential and powerful figure at court, she does not possess notable combat or strategic abilities. Her talents lie in singing, manipulation, and deception. She is a skilled liar who uses charm and scheming to get what she wants. Throughout the series, the Duchess shows little to no development or remorse for her actions. After the death of her daughter Charlotte, her immediate concern is that the suicide will be a stain on the family's reputation, not genuine grief for her child. She remains a selfish and conniving antagonist until the end, when she becomes one of the first nobles to flee Versailles during the Revolution, prioritizing her own fortune and safety above all else.