She is the daughter of Ysabell, Death's adopted daughter (introduced in The Light Fantastic) and Mort, who was briefly his apprentice (in Mort). At the end of Mort they leave Death's domain 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 (Death could not save them, only grant them eternal life at his Domain, which they refused). After graduating — and despite technically being the current Duchess of Sto Helit — she begins a teaching career, first as a governess (in Hogfather) and then as a schoolteacher (in Thief of Time). She insists on being addressed as "Miss Susan". She was 16 when first encountered in Soul Music and, since the novels take place roughly in real time, was roughly 21 during her final appearance to date in Thief of Time.
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 in Mort, and which glow when she is angry. In Soul Music, she complains that, as Death's granddaughter, she should have been born with better cheekbones. She also has a Look (always capitalised) which can be rather disconcerting, even to those in authority, such as her grandfather and the headmistress of the school in which she teaches.
Latterly, she accepts that she is part of the same world as the Hogfather and the Tooth Fairy—she just wishes she wasn't. Indeed, in Hogfather, she is seen to be making an immense effort to remain 'normal', carefully limiting her use of her inherited powers and forcing herself to act and act in as 'human' a way as possible. She mentions having great difficulty in rembering things which other humans take for granted, such as the purpose of doorknobs, the way to fall asleep and how to forget things, but argues that if she were to abandon this struggle and give herself fully to her supernatural side she would lose her humanity. However, in Thief of Time, though still resentful of having been born Death's granddaughter and attempting to avoid too much contact with the supernatural world, she appears to have accepted and even embraced her heritage: she admits, albeit grudgingly, that she is not completely human, frequently uses her powers to provide teaching aids, and views herself as an exception to the rules of normal human society, as evidenced by her willingness to investigate the personal papers of her employer, Madame Frout, without her knowledge.
In following with her practical outlook on life, Susan does not have a great deal of patience with people who behave in an inefficient or unfocussed fashion. She also has a certain distrust for philosophy, disliking its attempts to oversimplify an inherently complicated world. She has demonstrated the ability to retain a calm, guarded and analytical mindset even in the most extraordinary of situations; for example, when caught in a sort of distortion of reality upon approaching Jeremy Clockson's glass clock, instead of being overcome by fear or shock, she simply begins to consider whether or not what she is seeing around her is actually real.
She appears to have a preference for the colour black, in that most of her childhood drawings were done in this colour (mentioned in Soul Music). This fixation is not as extreme as that which her mother possessed for the colour pink, and may in fact be a characteristic inherited from Death, who made most of the decorations for his Domain black. She does, however, possess an intense fixation on chocolate (provided there is no nougat in it), one about which she is very defensive.
Another aspect of her personality which may stem from her relationship with Death is the fact that she is, by her own admission, "not very musical" (her grandfather also possessed a distinct lack of talent in the field of music, making several unsuccessful attempts to learn how to play the violin). However, this may be the result of her aforementioned upbringing in which imagination and creativity were not encouraged to any degree, making it more of an environmentally instilled trait than an inherited one.
Susan has, on several occasions, demonstrated rather unusual attitudes toward school and education in general: In Soul Music, she cites school as one of the greatest obstacles to education, and in Hogfather, she compares being educated to having a communicable sexual disease, in that "It made you unsuitable for a lot of jobs and then you had the urge to pass it on".
She has expressed a fondness for children, as long as they are "not raw", meaning that although the idea of having children herself does not appeal to her, she enjoys spending time with those of other people. She also demonstrates an insight into the more disturbing aspects of childhood nature, as she enjoys listening to the sound of children playing while she is reading, but always positions herself at such a distance that she is unable to hear what the children are saying, lest she find it unsettling. Because of this insight, she has a knack for putting problem children (such as those who try to get around her rules) in their place.
Her approach in other areas is also unusual. For example, in her role as a governess she has found that her charges' reading progress has been greatly enhanced by using interesting books which are slightly too difficult for them, and which therefore present something of a challenge. Parents may, however, have reservations about her choice of General Tacticus' Campaigns as a reader, since it may be argued that the ability to spell 'disembowelled' is not necessarily needed by children under ten.
As a schoolteacher she is sufficiently successful to have parents clamouring to have their child included in her class. Her approach to history and geography, often subjects which children find rather dull, has particularly captured her class's attention. The occasional need to remove from their children's clothing dried-in bloodstains or ground-in swamp mud is generally seen by parents as more than compensated for by the broad education being received—a child's description of one of the classic battles from Ankh-Morpork's long history, for example, might be sufficiently vivid and detailed to make the parent think that the description could not have been improved upon if the child had actually seen the battle at first hand (which in fact they have, given the flexibility of time and space Susan has inherited from her grandfather).
Her role as a caregiver to children combined with her no-nonsense style and almost magical flair for stick-like weaponry (e.g. her grandfather's scythe, field hockey sticks, a fireplace poker) could be seen as a punnish pormanteau of Mary Poppins who also had a very similar attitude (but actively encouraged positive imagination where Susan tends to say things like "Real children don't go hoppity-skip unless they're on drugs."), took children on magical adventures (Susan does so to teach geography), introduced children to supernatural creatures (Susan employs such beings as guest speakers) and had a magic umbrella (which, while not used as a blunt object, was magical much like Death's scythe). Neither character openly acknowledges their magical powers. In fact, both tend toward being actively insulted when reminded of their use. The link between Susan and Mary Poppins is explicitly alluded to in Hogfather, when Susan states that "She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps, she'd beat herself to death with her own umbrella". In 'The Art of Discworld', Paul Kidby draws Susan in Edwardian clothing, which he feels fits well with her job as a nanny in Hogfather.
Susan has inherited Death's perfect memory, which includes the ability to "remember the future" and an instinctive knowledge of the nature of Death's work, which gives her considerable assistance during her brief time doing her grandfather's job in Soul Music. In order to keep her from going insane, her minds 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. In Hogfather, she states that the fragmented nature of such foresight makes the power largely unhelpful.
Like Death or experienced witches, Susan can make herself completely unnoticeable to humans if she so chooses, though some people, such as Albert or Mustrum Ridcully, who are used to such things, are still able to see her if they concentrate and she is not actively trying to avoid their specific notice. During her time at Quirm College, she unconsciously used this power to avoid being asked questions in subjects in which she had no interest, such as Literature and History. Unfortunately, this caused the teachers of these subjects to believe that Susan was not attending their classes, and she was reported to the headmistress on several occasions. However, this did not pose much of a problem, since she would merely use her power to erase the headmistress' memory of her supposed misconduct.
She 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: in Soul Music, she is able to acknowledge the enormous size of the rooms in Death's Domain, whereas most humans can only deal with the incomprehensible vastness by pretending it is not there; Hogfather reveals that she shares the ability of witches, wizards and young children to see things which are invisible to most people, such as tooth fairies, monsters and bogeymen; in Thief of Time, she is able to identify the historical anomalies created by the History Monks' reconstruction of the shattered timeline. The latter novel also reveals that Susan possesses a sort of hyperawareness of the world around her, in that she is "aware of every step of the rocks beneath [her] feet and the stars overhead".
On various occasions, unusual circumstances have rendered some or all of Susan's powers useless: In Hogfather, she becomes completely human and finds herself bereft of her inherited gifts upon entering the Tooth Fairy's castle. This is because the laws of reality within the castle are based on the beliefs of children, which do not include a concept of death; hence the part of Susan which is derived from Death does not exist there. In Thief of Time, Susan finds herself unable to walk through walls whilst time is being held in stasis. She speculates that the energy of the glass clock which is holding time static is in some way interfering with her power.
Due in part to her education at Quirm College for Young Ladies, Susan possesses various practical skills, some of which contribute to her success as a governess and later a teacher: she is accomplished in the fields of Mathematics and Logic (priding herself on her ability to mentally calculate the square root of 27.4), is multilingual, and has considerable knowledge of Discworld culture. She is also a competent swimmer, being versed in four swimming strokes and several lifesaving techniques.
At the beginning of Soul Music, Susan is studying at Quirm College. When Death takes a holiday from his work in an attempt to forget his more troubling memories, a metaphysical vacancy is created in the post, and Susan is 'sucked in', as Albert puts it, forcing her to temporarily assume the role. At first, she desires to use her power to help humanity, but as the book progresses, she comes to realise that she is powerless to interfere, and discovers the inevitability of death and destiny. Much to her relief, she is released from the position following the return of her grandfather, who breaks the rules to save the rock star Imp Y Celyn and his band.
In Hogfather, the Auditors employ Mr Teatime, an assassin who attempts to destroy the Hogfather using teeth found at the Tooth Fairy's castle to control children. Death cannot enter the Tooth Fairy's castle because it is constructed from the imaginations of children, who have no concept of death, and therefore sends Susan to foil their plan instead. After removing all of the Auditors' human agents from the Tooth Fairy's country, Susan saves the Hogfather from being destroyed by a group of Auditors who have taken the forms of dogs. She later kills Mr. Teatime, by impaling him with the poker she uses to threaten monsters.
Thief of Time sees Susan once again in conflict with the Auditors, who are now attempting to stop time, so they can catch up with the paperwork. She is forced to find the son of Time, who Death cannot influence or even see, because—being only "mostly human"—the boy is not subject to death. She is aided in her mission to stop the Auditors by Lu-Tze (a philosopher) and Unity (a former Auditor who has become human).
She appears in the computer game, Discworld II: Missing Presumed...!?.