Mort and Ysabell are fictional characters, a young married couple in Terry Pratchett's Discworld series. Although they have appeared in only two of the almost forty Discworld novels (save for a brief death scene in Soul Music), their presence looms large within the series's grand story arc, particularly their roles as parents to the main character Susan Sto Helit. Mort and Ysabell are also referenced to in Guards! Guards!, though not by name.
Ysabell is the adopted daughter of Death, who saved her as a baby when her parents were killed in the Great Nef desert (no explanation has been given as to why he did this; Ysabell said that "He didn't feel sorry for me, he never feels anything... He probably thought sorry for me."). When first encountered she is a sixteen-year-old girl with silver hair and silver eyes who, it transpires, has been sixteen for thirty-five Discworld 'years' (no time passes in Death's Domain). During her encounter with Rincewind (see below), her behaviour is sufficiently flamboyant as to cause him to believe she is "bonkers". When Mort first encountered Ysabell, he was given the impression of "too many chocolates". She also has a fixation for the colour pink.
Ysabell first appeared in The Light Fantastic, where she met Rincewind, and was surprised to learn that he was not actually dead. This situation might not have lasted very long if the Luggage had not intervened. During the events of Mort it became clear that Ysabell was competent in carrying out the work of her father including The Duty and 'doing the nodes'. Before Mort arrived she shared her home with Albert, Death's manservant.
Mort, short for Mortimer, is the title character in Mort. He is first seen as the overly-thoughtful son of a farmer in the Octarine Grass Country, near the Ramtops. Having proved himself unworthy as a scarecrow he is chosen by Death to be his apprentice. Mort is described as being very tall and skinny, with muscles like knots in string. He has a shock of bright red hair, and walks as if he is made entirely of knees. Mort starts off at the bottom, learning to accept his position while mucking out the stables, and trying to ignore Ysabell, Death's adopted daughter. When Death feels in need of a break, Mort takes over The Duty. Unfortunately for Mort, his feelings for a teenage princess get in the way of his job and he starts off a chain reaction of events by impulsively preventing her assassination. Reluctant to tell his master about his gaffe, he tries various unsuccessful methods to fix the situation.
After fighting and losing to Death, Mort was given an extra lease of life when the Grim Reaper turned over his Lifetimer. This allowed him to stay in the world of the living.
After the events of Mort, Mort leaves Death's service and marries Ysabell. The couple are given the title of Duke and Duchess of Sto Helit, and later become the parents of Susan Sto Helit. They subsequently meet their end after a freak accident sends their carriage plunging into a ravine, as revealed in Soul Music. They had turned down an offer from Death to extend the duration of their existence on the grounds that it wouldn't be the same as actually lengthening their lives.
In "The Light Fantastic", Rincewind overhears Twoflower teaching the Four Horsemen of the Apocalypse (Death, Famine, Disease and War) to play bridge. At one point, one of the horsemen calls Death "Mort", but we later learn that the only people in the room (other than Twoflower) were Death, Famine, Disease and War.