In Terry Pratchett's Discworld novels, Susan Sto Helit is the daughter of Mort, Death's apprentice, and Ysabell, Death's adopted daughter. She has inherited a few traits from Death, such as the ability to disregard things like time and matter, and if she doesn't want to be seen it will be very hard to spot her. Her parents were worried that the influence from Death would be too strong, so they gave her a very sensible education at a boarding school. Unfortunately, a sensible education doesn't make much sense on Discworld where Death, the Tooth Fairy, the Hogfather and other anthropomorphic personifications are all real.
Susan often wears black, and her most unusual characteristic (apart from her being able to walk through walls) is her white hair with a black streak in it, which seems to unravel itself as it sees fit. She gets a job as a governess and shows children how to really deal with bogeymen and monsters under their beds. Later on she works as a teacher and gives the class graphical explanations of history and geography.
Susan has had to take over the family business (see Soul Music) when her grandfather has gone missing, although of course she tries to put a slightly more human touch to death. Death also asks for her help in situations where he can't intervene directly himself (even anthropomorphic personifications have rules to follow, after all), and Susan grudgingly steps up to the job. (See Hogfather and Thief of Time.)
Susan is a bit of an outsider. She is forced to act alone when Death asks for her help, aided only perhaps by Death's white horse Binky and the Death of Rats, but they aren't much of a company. Being not quite human sometimes makes it hard to make friends, as she gets frustrated by their lack of perception. Susan sees things that are actually there, which is much harder than seeing things that aren't there, as most people do. Her upbringing made her believe that the world should be a sensible place, and this notion makes life hard for her on Discworld.
Susan can be found in the following Discworld novels by Terry Pratchett; Soul Music (1993), Hogfather (1996), and Thief of Time (2001).