Throughout the story, Lucy and Polly serve as foils for one another, sharing very similar circumstances but ultimately responding to those same pressures in very different ways. Polly is the daughter of Peachum, who serves the criminal justice system by offering up other criminals for prosecution. Lucy is the daughter of a prison warden, Lockit, who works alongside Peachum to profit from crime in a legal manner. Both young women love the same man, Macheath, and both think of themselves as his true wife.
Ultimately, however, differences emerge between these two characters. While Polly is willing to use her wiles to claim Macheath for herself, Lucy shows a much darker side, attempting to murder her rival. Polly's more sympathetic nature is revealed by her inability to comprehend just how far Lucy is willing to go in order to bring about her own "happy ending." When Lucy offers her some "strong waters" or liquor, Lucy responds innocently:
All this wheedling of Lucy cannot be for nothing.—At this time too! when I know she hates me!—The Dissembling of a Woman is always the Fore-runner of Mischief.—By pouring Strong-Waters down my Throat, she thinks to pump some Secrets out of me.—I’ll be upon my Guard, and won’t taste a Drop of her Liquor, I’m resolv’d.
Suspicious of Lucy's feigned friendliness, Polly imagines that her rival intends to get her drunk in order to "pump some Secrets" out of her, little suspecting that the liquor has in fact been poisoned. At this moment, Polly proves herself the more virtuous young woman.
