Polly is the only character who undergoes any significant change in the course of the play. When the plays opens, she is a young, naïve girl who has fallen in love. She is initially horrified by the criminality of her new husband, but gradually she accepts the circumstances of Macheath’s business and even agrees to lead the gang in his absence. By the time Macheath has escaped from jail, Polly has been coarsened enough to try to trick Lucy into revealing where Macheath is hiding.
Polly’s relationship with Macheath causes the change within her character. She initially experiences his world of depravity and criminality with horror. But Polly eventually accepts the brutality all around her and helps to make Macheath’s thieves accept her as their new boss after Macheath tells them he has to leave. At the jailhouse with Lucy, Polly exhibits a toughness that contrasts her perceived sweetness. This toughness belies the jealousy that lies beneath, and it displays a virtuous girl that has become cruel.
Polly is unwavering in her love for Macheath. She loves him at their wedding, when he flees and gets arrested at a whorehouse, and even after she learns that he has another woman. This love creates a strong conflict with the self-interest that motivates the plot of the play. Polly’s love especially conflicts with her parents’ self-interest, and she represents the possibility of something bigger than self-interest.