Mary-Jo Cassidy is the older sister of both Phil Cassidy and Louise Cassidy-Williams and lives in a home in Little Havana, Vice City. Mary-Jo is envious of her sister Louise and is desperate for attention from anyone. She appears to have a crush on Victor Vance, although this is not elaborated on.
She is, however, very concerned when her sister is kidnapped by both Marty Jay Williams and later by Armando Mendez. During the mission, Light My Pyre, she says to herself "one day it will be her" and is adamant in telling Vic that Armando wanted her as Latin men love women of her type. Following the death of Louise, it is implied that Mary-Jo becomes the guardian of Mary-Beth Williams, Louise's daughter.
Her relationship with her brother Phil is not elaborated on.