Milady operates as Cardinal Richelieu's primary antagonist, functioning as a spy and assassin driven by financial gain and personal vengeance. Her past includes marriage to the musketeer Porthos, which ended when he uncovered her criminal history and attempted to execute her by hanging; she survived. This criminal past involved stealing sacred church artifacts alongside a priest during her youth, resulting in a fleur-de-lis brand marking her as a convicted criminal—a secret she guarded fiercely.

After surviving the hanging, she adopted the identity of an English noblewoman, marrying Baron Sheffield and bearing his son. She later poisoned Sheffield to inherit his wealth, disguising his death as illness. Serving Richelieu, she stole Queen Anne’s diamond studs to provoke political scandal, though this plot failed. She then orchestrated the Duke of Buckingham’s assassination by manipulating the Puritan soldier John Felton, exploiting his grievances and fabricating tales of persecution before abandoning him post-murder.

Her conflict escalated when she poisoned Constance Bonacieux (adapted as Juliette in some media), murdering the protagonist’s love interest—an innocent woman who trusted her—to target him directly. Pursued by the musketeers, Athos (her former brother-in-law), and a mysterious executioner from her past, she faced a vigilante trial. Charges included multiple murders, espionage, theft, and bigamy for marrying Sheffield without annulling her union with Porthos. The trial concluded with her execution by beheading.

Physically depicted as an anthropomorphic black cat—reflecting stealth and espionage within the adaptation’s animal-based design—she leveraged beauty and charm for manipulation, frequently adopting false identities to achieve her objectives.