She is the daughter of Mort and Ysabelle. Ysabelle was the adopted daughter of Death and Mort was, briefly, Death's apprentice. They leave Death's ambit and become Duke and Duchess of Sto Helit, Susan is their only child. She is first introduced as a sixteen year old pupil at the "Quirm College for Young Ladies" in Soul Music, shortly after the death of her parents.
Despite her tenuous relationship to Death, she is often called his granddaughter and she refers to Death as grandfather. Because of this link she has certain abilities, she can "walk through walls and live outside time and be a little bit immortal." As a career she teaches.