She and Valmont were lovers before because he was nephew of her friend afterwards she had affair with the Counrt thus left Valmont into notorious womanizer who finding the challenge too easy from Merteuil. Two of their victims were Madame de Tourvel who is wife f potilction who is remian virgin and faithful to her husband and Cecile who had been betrothed to Merteuil's former lover but she was in love with her music tutor Danceny (who also feelings for her) and Merteuil suggests Valmont to played with Celice as begun seduced her which led loss of Celice's virginity. when Cecile confessed to Merteuil about her exeprince for losing virginity to Valmont by guilt that if her mother found out as Merteuil encourages her to married her lover means Danceny as male mirtess.
Merteulli's next victim was Madame de Tourvel who stayed in Valmont's aunt while her husband was away
The character Kathryn Mertueli of the 1999 film Cruel Intentions is based on Marquise