Susan is the daughter of Ysabell, Death's adopted daughter and Mort, who was briefly his apprentice. She's also a schoolteacher who insists that her students address her as Miss Susan.
Susan's most obvious character trait is being sensible, an attribute carefully cultivated by her parents as a counterbalance to the influence of her grandfather. Initially, this manifested itself as a refusal to admit the supernatural side of the world (beyond basic magic) even existed. This is not surprising, given the fact that her parents had informed her from a very early age that there were no such things as the Soul Cake Duck, the Hogfather or the Tooth Fairy. It has even been suggested that her parents chose to name her Susan specifically because of the sensible and conventional connotations of the name. This upbringing has instilled in her a certain lack of romanticism; her personal appraisal of "a poem about daffodils" is, "Apparently the poet had liked them very much." She can be relied upon to keep her head in a crisis, something she tends to view as a character flaw.
Physically, Susan is a slightly built young woman often described as attractive in a skinny way. Her hair is pure white, but with a streak of black running through it from end to end. Although its natural state is a frizzy mass reminiscent of a dandelion, it rearranges its style of its own accord depending on Susan's mood; it tends to form a tight bun while she is working. Susan has a very striking physical presence, which both Lu-Tze and Lobsang Ludd have remarked upon, and despite her relative youth, she projects a sense of great age. She possesses a birthmark on her cheek that shows itself only when she blushes; it consists of three finger-like lines that were left on her father when Death slapped him, and which glow when she is angry.
Being Death's adoptive granddaughter, Susan has inherited some of his abilities: She can walk through walls and live outside time and be a little bit immortal. As well as being able to find any living creature, Susan can sense whether or not an entity is human. She has also inherited Death's perfect memory, which includes the ability to "remember the future" and an instinctive knowledge of the nature of Death's work. In order to keep her from going insane, her mind represses much of the knowledge provided by this memory, but even so, parts of it occasionally permeate her consciousness, generally manifesting as premonitions or intuitive insights into forthcoming events. Like Death or experienced witches, Susan can make herself completely unnoticeable to humans if she so chooses. She also possesses a unique perspective on life, in that she lacks the normal human ability to ignore things which do not fit in with a logical world-view.