Foil to the earnest Lady Chiltern, Mrs. Cheveley is the play's femme fatale: bitingly witty, fabulously well dressed, cruel, ambitious, opportunistic, and, above all, duplicitous. Repeatedly the play describes her as the product of "horrid combinations," evoking her dangerous deceitfulness. Thus Lady Basildon recoils from her "unnatural" union of daytime genius and nighttime beauty; later, Cheveley appears as a "lamia-like" villainess—that is, part woman and part snake. Whereas Lady Chiltern is pure and undivided, Mrs. Cheveley is defined by deception, artifice, and falsehood.
Cheveley returns from Vienna as a sort of ghost from the past, at once an old enemy of Lady Chiltern's from their school days, the traitorous fiancée of the young Lord Goring, and a disciple of the deceased Baron Arnheim, Sir Robert's seductive corrupter. Even more than Sir Robert, she fiercely subscribes to Arnheim's philosophy of power and gospel of wealth, treasuring the domination of others above all. Thus she unscrupulously wreaks havoc in the Chiltern's married life to secure her fortunes and dismisses marriage as a mere transaction. Thus, within the moral scheme of the play, she stands opposed to the sentimental notions of conjugal life embodied by the Chilterns and Lord Goring.
With this in mind, Mrs. Cheveley's undoing in Act III avenges her crimes against the conjugal household. Called to account for a past crime, she finds herself trapped for a stolen wedding gift—the diamond brooch—by her ex-fiancé. The poetic justice in her arrest is clear. Moreover, this undoing also unmasks her as a monster. Once trapped by Lord Goring, Cheveley dissolves into a "paroxysm of rage" her loss of speech giving way to an agony of terror that distorts her face. For a moment, a "mask has fallen," and Cheveley is "dreadful to look at." Her veneer of wit and beauty thus give way to the hidden beast.