Rosalie is 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.
