Mary-Beth is the newborn baby of Louise Cassidy-Williams and Marty Jay Williams. She appears several times in GTA VCS, often in her mother's arms. Like her mother, Mary-Beth is threatened with violence by her father.
She is orphaned after her father is killed by Victor Vance and her mother dies from wounds caused by Armando Mendez. Whilst dying, Louise asks Vic to make sure Mary-Jo Cassidy takes care of her, so it is assumed that she becomes Mary-Beth's legal guardian.