Who is Susan Sto Helit? Susan Sto-Helit, or Susan Death, is one of the characters from Terry Pratchett's Discworld novels. She appears in Soul Music, Hogfather, and Thief of Time - and her parents' story is told in Mort. She initially had black hair, with a single white streak, which was completely unruly (and apparently alive). As an adult, in the later books, she has white hair with a black streak - which tends to reflect her mood, and can curl itself neatly into a bun.
She's a formidable and practical young lady, who has little patience for nonsense and sees things as they are. Her grandfather is Death (and apparently there are magical genetics at work, because despite only being her adoptive grandfather, she's inherited plenty).
Susan is introduced as a young girl in Soul Music, growing up in boarding school (the Quirm College for Young Ladies) after her parents died in a carriage accident. Death decides to go on holiday and try and learn how to forget. She gets called on by the metaphysics of the universe to take over for Death (and has to come to terms with that!) and gets caught up in the story of Imp Y Celyn (who looks a bit elvish around the ears), the Discworld's first rock and roll star. He's supposed to die, but the music takes him over, causing a great deal of trouble!
In Hogfather (now a film), a rather unusual assassin, Mr. Teatime (that's Te-ah-tim-eh), sets out to kill the Hogfather (the Discworld version of Father Christmas - a lot closer to the original myths, with his pigs and beans!). Death takes over his role in an attempt to keep the belief alive, and Susan gets dragged in to try and fix things. Her Grandfather sends her to the Tooth Fairy's home, which is built on children's beliefs, and is a place where death (and therefore Death) cannot exist. Here, she's about 21 years old, a governor for two children, and beats up the bogey men with a poker.
In Thief of Time, it's not so much Susan's story - more that she is dragged into the tale of Lobsang Ludd, son of Time, in a fight against the Auditors of Reality (who hate messy things like life - and were also responsible for hiring Mr. Teatime in Hogfather)