Mary Waverly, youngest daughter of the Waverly family, enters Lynn Russell’s life as a stepsibling following her mother Magdalene’s pursuit of a union with the Russells. Alongside her brother Thomas, Mary relentlessly bullies Lynn, fueled by envy and Magdalene’s obsession with attaining noble standing. She devises calculated schemes to sabotage Lynn’s social ascent, though fleeting remorse surfaces when her antics spiral beyond control.

Her bond with Prince, a Persian cat, mirrors her inner conflict. Initially exploiting the feline as a pawn in her plots, she coldly severs ties when he gravitates toward Lynn, prompting his adoption by her rival. This act highlights both her vindictiveness and the fragile insecurities rooted in her family’s fractured aspirations.

The manga concludes her arc with a retreat to Japan under Lynn’s grandparents’ care after the Marble Mansion’s loss and Magdalene’s collapsed ambitions, signaling her exit from antagonism. The anime diverges, recasting her as a persistent foe in later arcs. Allying with Vivian Spencer, a competitive equestrian, she schemes to thwart Lynn’s quest for the Lady Crest, her bitterness fueled by her father’s rejection of Magdalene’s marital overtures.

Her anime trajectory charts a path of steadily intensifying malice, peaking as the "final villain" who deploys blackmail and sabotage to exile Lynn from school. Subtle hints of internal struggle emerge—brief hesitations, unintended fallout from her machinations—yet her story closes without absolution, framing her choices as products of relentless familial expectation and society’s corrosive allure.

Portrayals in films, OVAs, or spin-offs remain undocumented beyond the core manga and anime series.