The Duchess of Monmouth is a young, beautiful woman married to the Duke of Monmouth, an older, boring man who is not well-matched to his wife’s vivacity and intelligence. As such, the Duchess, like many in the high society she exists in, openly engages in extramarital affairs. Near the end of the novel, Dorian becomes a target of her interest. However, unlike the other women Dorian interacts with throughout his sinful and debaucherous life, the Duchess possesses a strong personality that makes her less vulnerable to Dorian and Lord Henry’s corrupting influence. The Duchess is one of the few characters who can match Lord Henry’s wit, and her presence on the page is often defined by long passages of quick, scintillating dialogue. Lord Henry seems delighted when engaging with her in public and admits to her intelligence, but in private he confesses that he finds her clever tongue irritating. Lord Henry derives the greatest pleasure from influencing people, so the Duchess’s propensity for easily and wittily interrogating and rejecting his worldview is perturbing to him, as he cannot exert control over her to the same extent as he does over Dorian and others in his social circle.

Additionally, the Duchess of Monmouth challenges Lord Henry’s sexist views of women. Throughout the novel, Henry makes a number of flippant and misogynistic statements about women, such as that they are a decorative sex, that they never have anything important to say, and that, despite having made progress in their social equality, still secretly desire to be dominated by men. The Duchess of Monmouth defies Henry’s views by matching him in intelligence and wit, engaging him in conversation and debate in a way that no other character in the novel can. While the Duchess is a relatively small character, her presence at the end of the novel, which mostly revolves around her ability to go toe-to-toe with Henry, further strengthens the reader’s growing sense that Lord Henry is not the purveyor of truth that Dorian believes him to be. Crucially, the Duchess contests Lord Henry’s assessment that beauty is the most important virtue, claiming that she feels he puts too much value on physical appearance. Her remark throws into question the core of Henry and Dorian’s worldview and casts doubt on the validity of their philosophies and behaviors.

However, although the Duchess is strong-minded and less prone to bad influences than other characters in the novel, she is not unaffected by Dorian’s otherworldly beauty. Subtle comments in her exchanges with Henry suggest she is romantically interested in Dorian, and later, she openly admits to Henry that she’s considering whether she should proposition Dorian. Fortunately, James Vane’s sudden death distracts both Dorian and her from the romantic missive she sent, and Dorian's own end soon later saves the Duchess from coming into greater contact with his pernicious nature.