Duke Mortimer of Sto Helit is a recurring character in the Discworld series and the main character in the novel Mort. His given name is Mortimer, but he prefers to be known as Mort. For a brief time he was an apprentice to Death and temporarily carried out The Duty on his behalf.
He is described as being very tall and skinny, with muscles like knots in string. He has a shock of bright red hair, and looks as if he is made entirely of knees.
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.
Mort grew up on a farm in the Ramtop Mountains. Mort's life in the mountains was highly unremarkable until, one Hogswatch eve his father took him to Sheepsridge to find him an apprenticeship. Mort was largely unsuccessful in finding an apprenticeship at the job fair. That is until at the stroke of midnight when a dark, hooded figure arrived on a white horse to offer him an apprenticeship "ushering souls in to the next world".
Mort accepted the apprenticeship out of a desire to please his father and left with his new employer, Death. Whilst living in Death's Domain Mort met Ysabell, Death's adoptive daughter and Albert Death's servant.
Mort's duties initially consist of mucking out the stables and general cleaning duties. However, he is eventually invited to assist with what Death refers to as The Duty. This involved assisting with the death of the King of Sto Lat, after his assassination by the Duke of Sto Helit.
After Mort's initial experience with The Duty, Death decides to allow Mort to complete The Duty by himself. Initially feeling very awkward, Mort soon gets to grips with the job. However, he soon makes the mistake of altering causality by saving the life of Princess Keli on whom he has developed a small crush. The resulting damage to the timeline forces Mort to enlist the help of Ysabell in repairing the damage he has done.
Ysabell who initially shows a strong dislike to Mort eventually grows to love him. The adventure they share together brings them closer together and they eventually get married and have a daughter whom they name Susan
Mort is a well-meaning, but rather dim-witted young man. He has an enquiring mind and is eager to please. He is a moral person and found that the job of ushering souls in to the next world did not suit him, as he rather regretted the unfortunate necessity of having to kill people.
He is also shown to be a protective father, as he was initially unwilling to allow Susan to visit her grandfather, but grudgingly allowed a single visit that became more. When she was three, Mort decided to stop the visits and cut off all contact with Death, even going as far as taking away the My Little Binky playset Death had gotten for his granddaughter.