Mary-Jo is the obese sister of Phil Cassidy and Louise Cassidy-Williams and lives in Little Havana, Vice City. She is the envious of Louise and is desperate for attention from anyone. She appears to have a crush on Victor Vance, although this isn't elaborated on. However, she is shown to be very concerned when her sister gets kidnapped by Marty Jay Williams and Armando Mendez. During the mission Light My Pyre, she tells herself "One day it will be me" and is adamant in telling Victor that Armando wanted her, as Latin men love women of her type. It is implied that she takes care of Mary-Beth Williams after her sister's death.
Her relationship with Phil isn't elaborated as she isn't mentioned by him during the events of Grand Theft Auto: Vice City, Grand Theft Auto: Liberty City Stories, and Grand Theft Auto III.