Nathalie Sancoeur is a major character of Miraculous: Tales of Ladybug & Cat Noir, serving as a recurring character of Season 1, the final antagonist of Season 2, the secondary antagonist of Seasons 3 and 4, a major anti-hero of Season 5, and a minor character of Season 6. She is also a major antagonist of Miraculous World: New York - United Heroez, a supporting antagonist of Miraculous World: Shanghai - The Legend of Ladydragon, a minor character of Miraculous World: Paris - Tales of Shadybug and Claw Noir, and the tritagonist of Miraculous World: London - At the Edge of Time.

She is a former treasure hunter of magical objects and Gabriel Agreste's assistant who manages his fashion work, his mansion, and takes care of his son, Adrien.

In "Catalyst", which is part of the "Heroes' Day" special, Nathalie was voluntarily akumatized by Hawk Moth into Catalyst, a power-enhancing supervillain.

In "Mayura", in order to save Hawk Moth from the French Miraculous superhero team, Nathalie used the Peacock Miraculous. When the Peacock Miraculous was inhabited by Duusu, she became Mayura, a peacock-themed supervillain with the power of Emotion who helped Hawk Moth by creating allies for akumatized villains.

In "Ladybug", Nathalie was reakumatized into Catalyst in an attempt to help Scarlet Moth, but due to being ill, she was de-akumatized shortly after.

Throughout Season 4, due to using the damaged Peacock Miraculous, Nathalie was left very weak and stuck in bed, but was eventually well enough to get back to work, with the aid of robotic leg braces that help her walk.

In "Evolution", after Gabriel failed to fix the Peacock Miraculous by giving the recipe to do it to his younger self, in that way preventing both Émilie and Nathalie from using the damaged Miraculous and getting sick, all because of his fixation to get Ladybug and Cat Noir's Miraculous, Nathalie, feeling betrayed by him, decided to break ties with him and stop helping him, leaving Gabriel frustrated that she had abandoned him for his failure.

In "Passion", with Adrien in risk of becoming alone due to both hers and Gabriel's terminal situation and to honor Émilie's wishes, Nathalie let herself to be akumatized by Monarch into Safari, a hunter-themed supervillain with the power to never miss her target and received the power of the Goat Miraculous superpower through her Alliance ring. She also decided to take the Ladybug and Cat Miraculous herself in order to prevent Gabriel from using them to mold the universe into his own madness.

In "Representation", Nathalie's illness from the Peacock Miraculous got worse, as there were visible eye bags under her eyes and the red highlights on her hair turned white. Gabriel also took the other half of the double wedding ring from her, as he said that she was unworthy of upholding the legacy of the Agrestes.

In "Conformation", Nathalie attempted to do a video message for Adrien, but failed to do so. She also tried to stop Gabriel, but passed out, after Gabriel transformed into Monarch and attacked her with his cane. He took Nathalie to her room, where Ladybug was hiding. Once Gabriel left the room, Ladybug questioned Nathalie about where Adrien was. Nathalie informed her that he was safe and implored her to stop Monarch at all costs before she succumbed to her illness.

In "Re-creation", after Gabriel sacrificed himself and made the wish to heal Nathalie and to redeem himself, she recovered from her illness.

In "Miraculous London", after awakening from her coma, Nathalie confronted Bug Noire, in this way discovering her secret identity as Marinette, and learned about Gabriel's sacrifice, and helped the heroine release the imprisoned Kwamis while revealing Gabriel's plan and at Bug Noire's request, she decided not to tell Adrien about his father being Monarch or her role as Mayura to take care of Adrien.

In “Werepapas”, Nathalie has taken on the role of Adrien Agreste's legal guardian following the death of his father. This marks a significant shift in Adrien's life, as he is no longer under his father's strict control. Nathalie, who had always been loyal to Gabriel and cared deeply for Adrien, now steps up to ensure his well-being. Given her past as Mayura and her deteriorating health due to overuse of the Peacock Miraculous, it remains to be seen how she will handle this new responsibility and what it means for Adrien's future.