Mary Waverley is the youngest daughter of Magdalene Waverley and becomes Lynn Russell's stepsister after Magdalene's relationship with George Russell, the Viscount of Marble. Initially, Mary torments Lynn, frequently collaborating with her brother Thomas to destroy Lynn's possessions, spread malicious rumors, and isolate her socially. This antagonism stems from jealousy over Lynn's acceptance into the Russell family and Magdalene's active encouragement of her children's actions while excusing their misconduct.

Despite her antagonistic role, Mary occasionally exhibits hesitation or flashes of remorse, particularly when her actions inflict severe distress on Lynn. These moments suggest an underlying conflict between her desire for maternal approval and a nascent conscience. Her primary loyalty remains with her biological family, and she often participates in schemes to undermine Lynn's position within the household.

Later, during Lynn's enrollment at an elite boarding school, Mary forms an alliance with Vivian, another student. Their partnership focuses on sabotaging Lynn's efforts to earn the prestigious Lady's Crest award, fueled by shared resentment and rivalry. Mary's tactics escalate to blackmail, threatening to expose Lynn's non-aristocratic background unless she withdraws from the competition.

Mary's character development culminates after Lynn's victory in the Olympic qualifier horse race, where Lynn publicly extends forgiveness. This act, combined with the Waverley family's diminished influence following Magdalene's failed marriage plans, paves the way for Mary's reconciliation with Lynn. The resolution sees Mary acknowledging past misdeeds and accepting Lynn as part of her extended family.