Mathilde de La Mole is the daughter of the marquis in whose house Julien Sorel works as secretary in Paris: a nineteen-year-old woman, rich, proud, and bored, who decides to love Julien precisely because he is not noble.
Boredom is what moves her. At a ball, she recognizes that fortune gave her everything but happiness, and she measures her suitors by a strange yardstick: "I think only the death sentence honors a man." On the anniversary of the execution of the ancestor Boniface de La Mole, beheaded in 1574, she dresses in black. What impresses her about the story is that Queen Margaret of Navarre had her lover's head asked for from the executioner, and then buried it herself. Julien, a commoner and bold, seems to her capable of what the young noblemen who court her lack.
Her love is deliberate. The narrator says that Mathilde's enthusiasm "was a little deliberate," and calls her feeling "cerebral love." She summons Julien to her room by the gardener's ladder, soon regrets it, declares that she no longer loves him, and falls at his feet only when he pretends to court the Marechale de Fervaques. Pregnant, she confronts her father to marry him.
In the end, to save him, she negotiates with the vicar general Frilair. After the execution, she kisses Julien's head and buries it with her own hands, repeating the gesture of the queen of the legend.