Milady operates as Cardinal Richelieu's chief English spy and principal antagonist. Her origins trace back to Charlotte, a Protestant commoner arrested in youth and confined to a convent. Richelieu forged her a new identity and trained her in espionage under Rochefort, forging a deep-seated hatred for the French crown and aristocracy that drives her actions.

She wields an ultrasonic whistle to command animals, utilizing this skill for surveillance and attacks—most notably directing a parrot to spy on Queen Anne. Her pet monkey, Pepe, shares a complex bond, serving more as a companion than a mere instrument. Proficient with daggers, Milady executes multiple assassination attempts, targeting figures like d'Artagnan and Constance through methods including drowning and fire traps. Her deceptive tactics encompass faking death via poison antidotes supplied by Pepe and adopting disguises, such as a domino mask.

Within the television narrative, Milady functions as Richelieu's co-dragon alongside Rochefort, engineering scandals involving Queen Anne's diamonds to disgrace her. She relentlessly pursues d'Artagnan during his mission to retrieve the queen's necklace from England, deploying numerous schemes to intercept him. Despite her antagonism, her backstory and a solitary death scene where she prays for forgiveness introduce sympathetic dimensions.

The film sequel confirms her survival following the fortress explosion. She resurfaces as the primary antagonist, concealed beneath a golden mask, masterminding a plot to steal a treaty signed by Catherine de Medici that cedes southern France to Spain. Commanding henchmen like Pizzaro, she orchestrates the murder of flower seller Carina and frames d'Artagnan for the crime. Her final confrontation unfolds at Château de Joux, where she steals the treaty from Aramis and attempts escape via deltaplane. An attack by Jean's parrot causes her to crash; subsequently engulfed by an avalanche while struggling with d'Artagnan, she vanishes without confirmed death.