She is portrayed by Milla Jovovich.
Betrayal in Venice
She helps the Three Musketeers to get into Da Vinci's vault in Venice, obtaining one of the needed keys from an unspecified baron. She then meets with Arthos, a musketeer and her long time love interestm and both of them share a deep kiss.
While the musketeers still argue how to best evade the hidden traps inside the vault, de Witer sprints into the hallway, activating hidden shooting mechanisms. She evades the projectiles shot at her and manages to get through the corridor safely. Inside the vault, the musketeers get the plans they came for.
After escaping safely, the musketeers and de Winter meet at their headquarter, drinking to their victory. Milady asks Arthos what they will do now to which Arthos answers that they will return to France with the stolen plans. De Winter then tells them that she has other plans and reveals that she has put a narcotic into their wine, calling it "nothing personal". While the musketeers drop to the ground, they are able to see the their rival Buckingham arriving, with whom de Winter is working because he payed better. Buckingham then gloats over the musketeers before he and de Winter depart.
Plotting with Richelieu
On the way to Paris, she and her guards stay at a tavern where the leader of her guards, Rochefort, almost kills the young D'Artagnan, who plans to become a musketeer. D'Artagnan is saved by de Winter, who tells Rochefort that D'Artagnan is too pretty to die.
At Paris, she visits Cardinal Richelieu and tells him the news from England: that Buckingham is coming and built the war machine de Winter sold to him a year ago. Richelieu pays her for a job well done, which reveals that de Winter is in truth serving Richelieu. They are disturbed by King Louis, who arrives and asks de Winter of news from Buckingham, which shocks de Winter as the king normally isn't interested in things as politics. However, Louis specifies that he wants to know which colour Buckingham is wearing these days, which Richelieu comments with a knowing smile. After screaming at his advisors for advising him blue clothes when Buckingham wears green he storms out of the room, leaving Richelieu and de Winter to their plotting.
After Buckingham arrives at Paris, he and Richelieu meet to dicuss their peace treaty. While they talk, de Winter enters the Queen's rooms by jumping from the roof, hides some letters in her desk and steals a necklace out of her room, carefully avoiding traps to get to the necklace. She then meets with Richelieu, who tells her to travel to London with Buckingham and put the diamond necklace into his personal vault, the Tower of London. Before de Winter leaves, Richelieu tells her how tempting it has to be for a double-agent to play both sides. He tells her that her while he admires her, he would destroy her the moment he suspects that she turns on him. She tells him that she wants an assurance for her well-being in case Richelieu's treason is found out, so Richelieu writes her a document stating that the bearer did everything he did for the good of the state. Alongside Buckingham, de Winter then departs with Buckignham en route to England.
After the musketeers have been called to get the necklace back from Buckingham, de Winter is alerted by Rochefort and warns Buckingham about the musketeers. She briefs Buckingham about each musketeer and their prefered way of approach. To trick them, the musketeers change their way of approach and send D'Artagnan into the Tower in broad daylight. However, upon entering D'Artagnan is almost immediately arrested. When informed about that, Milady makes clear that she plans to leave, as she has no intention of watching Arthos die, who she thinks will be captured as well. De Winter leaves and D'Artagnan is brought before Buckingham, who asks him what he came for. When D'Artagnan remains quiet, Buckingham is surprised and asks whether the boy does not have any last insults for him, to which D'Artagnan replies by telling him to look out of the window. Upon doing so, Buckingham is horrified to see that the rest of the musketeers took over one of Buckingham's warships. They attack the tower, taking down an entire wall. To escape Buckingham, D'Artagnan jumps over the edge of the tower, boarding the airship and escaping London onboard. When asking why they are leaving when the necklace is still inside the tower, Arthos reveals that Milady never left the diamonds there, instead escaping the tower with them. He also reveals that the musketeers captured Milady while D'Artagnan was imprisoned. To do so they carried off her entire carriage.
Captured by the musketeers
As the musketeers prisoner, Milady tries to shoot Arthos with a pistol hidden inside her carriage, only to find out that Arthos has removed the gun powder. Arthos then tells her to hand over the necklace, elswise he takes it from her corpse. Arthos then brings her in the main room of the airship, where she meets the musketeers for the first time after betraying them in Venice. She recognizes D'Artagnan as the boy from the tavern and reminds him that she saved his life. She then asks Arthos whether he will kill her, which he confirms. When she asks him why he would kill her, but not Buckingham, Arthos replies that he doesn't hate Buckingham. De Winter draws the document written to her by Richelieu and puts in on the table. Arthos then forces her to go outside, near the edge of the ship. He aims his gun at her but before he can shoot her, she takes a step back, going over the edge of the ship and falling hundreds of feet into the ocean. Aramis comments that she died as she had lived - on her own terms. Arthos states that she did it for him, as he couldn't have lived with himself had he pulled the trigger.
At the end of the movie, it is revealed that Milady survived her fall into the ocean when she is fished out of the water by Buckingham, who is approaching France with a fleet, planning to get back what is his with interest.