Marie Rostain, mother of Isabelle Rostain and wife of Leon Rostain, belongs to the noble Rostain family residing in France during the Franco-Prussian War and Paris Commune. She and Leon have multiple children, including Isabelle, Geneviève (renamed Floriane in French adaptations), and Andréa. Occupying a prestigious position in Parisian high society, Marie initially embodies traditional values, prioritizing family reputation and social standing. This focus leads to her disapproval of her daughter Geneviève's relationship with a common piano teacher, reflecting her concern for maintaining social boundaries.

The Prussian invasion and defeat of Napoleon III's army shatter the Rostains' privileged existence. Forced to abandon their estate, Marie relocates to Paris with her family as the Prussians advance, thrusting them into the Siege of Paris. Confronted by widespread suffering and the city's defense, the family undergoes a significant shift. Marie actively participates in the resistance against the Prussian invaders alongside her husband and children, signaling a departure from her insular life and evolving engagement with the plight of common people.

Marie Rostain's character development ends tragically during the Prussian bombardment of Paris. She dies in the shelling in episode 11. Her death profoundly impacts her daughter Isabelle, who witnesses it. This loss fuels Isabelle's intense hatred for the war, crystallizing in her declaration, "I hate the war that killed mother." Marie's death becomes a catalyst for Isabelle's subsequent actions fighting the besieging forces and participating in the Commune uprising. It forms part of the broader tragedy befalling the Rostain family during the conflict and the violent suppression of the Commune.