Rosalie is Oscar's best friend. Rosalie is also the adoptive daughter of a commoner named Nicole Lamorlière. She is described as "a stereotypical good girl, sweet, obedient and timid". Her biological mother is the Duchess de Polignac referred to as Martine Gabrielle. Nicole tells Rosalie the truth when she is dying after being hit by a carriage. Rosalie swears that she will kill the person in the carriage that killed her mother. She tries to kill Oscar's mother, Lady Jarjayes, because she thought Lady Jarjayes was in the carriage, but Oscar stops Rosalie. Oscar decides to help her, and she teaches Rosalie the art of fencing, manners, history, and other courtly skills. Rosalie comes to admire and love Oscar greatly. Rosalie tells Oscar about Martine Gabrielle. At a party, Rosalie recognizes the Duchess de Polignac and tries to kill her, but Oscar stops her and tells her that killing her won't bring her mother back, and if she killed her, she would be executed. Later Oscar and André discover the true identity of Martine Gabrielle and her relationship to Rosalie.
Rosalie ambushes Polignac's carriage in front of Polignac's daughter, Charlotte, but Oscar stops her. After an incident where Charlotte loses her mind and commits suicide, the Duchess de Polignac goes to Jarjayes's Mansion and tells Rosalie that if she didn't come with her, she would tell the authorities that Oscar was the protector of the sister of Jeanne of Valois-Saint-Rémy, the mastermind of the "Affair of the Diamond Necklace". Rosalie decides to go; soon, she realizes that the Duchess de Polignac intends were to marry her to the Duke of Guiche, Charlotte's ex-fiancé and cause of her suicide. She escapes, and marries Oscar's friend, Bernard Chatelet and meets Oscar again during the Storming of the Bastille.
After the storming of the Bastille, the people burst into Versailles and take Marie Antoinette prisoner. Marie Antoinette and her family are taken to a prison, where Rosalie serves the queen as a servant, telling her all she knew about Oscar. Marie Antoinette, before the day of her execution, gives Rosalie a paper rose, and tells her to paint it with Oscar's favorite rose color.