When the first living thing existed, I was there, waiting. When the last living thing dies, my job will be finished. I'll put the chairs on tables, turn out the lights and lock the universe behind me when I leave.
Death, also occasionally called Mort by the other horsemen of the apocalypse, is the personification of death. Like most Grim Reapers, he is a black-robed skeleton (usually- he wears a "Born to Rune" leather jacket in performing in his band), carrying a scythe and, for royalty, a sword.
His steed is a great pale horse called Binky who is very much still alive. It is remarked that he tried riding a steed of flames and a skeletal one, but the former was constantly setting the stable on fire, and he got rid of the latter because he grew tired of "constantly getting off to wire bits back on". His hollow, peculiar voice is represented in the books by unquoted ; it is peculiar because since he is a tall skeleton, he has no vocal cords to speak with, and therefore the words enter your head with no involvement from your ears. It is often written without quotes, but as a quote.
As your Reaper, I often speak like this.
Death is not invisible. Most people just refuse to acknowledge him for who he is, unless he insists. Under normal circumstances, only those of a magical disposition, children and cats can see him, or allow themselves to see him. At one point, Death briefly took the place of an actor playing him in a play and was shocked to discover that when he walked onto the stag, all of the audience could see him as they were expecting Death to appear; it was stated that he was quite nervous at this as he is usually seen only by one person at a time and was not comfortable with so many people watching him. Death can of course ignore things like walls or magic spells that stand between him and his object: this is because he's much "realer" than they are. A castle might stand for centuries, but Death has existed for billions of years: to him, the walls of the castle are less substantial than a cobweb. However, he can only go where people believe in death, and can only see people who can die.
It is also mentioned that wizards, witches, and significant figures (e.g. kings) have the "privilege" of being collected by Death himself upon their death, rather than one of the lesser entities. Indeed, wizards and witches have prior knowledge of the time of their deaths and expect Death to keep to schedule. Most other deaths are collected by another functionary, but with the exception of "authorized" replacements for Death, there has only been one unauthorised "collection" described in the record books by anyone other than Death, attempted to take Arsenal Vetinari by the anthropomorphic personification of Scrofula. However, Death himself must collect some souls in order to keep the momentum of death going, worked out by a system described as the 'nodes'. As well as wizards and kings, he has shown up for numerous ordinary people, at least two dogs, a swan, and once for a tube worm (he used a one inch long scythe on it). He was present at the beginning and end of Time. He has also appeared even in situations where people might potentially die. Death has said that he does personally collect the souls of ordinary people. This suggests that Death's conception of an important person is not, necessarily, the same as a human conception. Possibly he is able to judge "significant" deaths based on an understanding of the broad sweep of history and chaos theory. He may also do ordinary deaths judging by the showyness of the death (such as a beggar being incinerated by a dragon, the first dragon-based death in centuries).
Death is efficient but not cruel, and sees his job as a necessary public service. His task is not to kill, but to collect. He harvests the old, worn-out souls of the dead so they may be properly "recycled" into fresh new lives. His is a necessary service as life force that isn't "recycled" properly tends to start producing unusual behavior
He is fond of cats (who can see him at all times) and curry (although he doesn't need to eat), which he often says is like biting a red-hot ice cube. He lives in an extradimensional realm called Deaths Domain. Within the domain, his home looks like a normal upper middle class Victorian house with a garden, (With little skeletal Garden Gnomes, as well as skeletal fish in a pond), is well-tended, but is predominantly black and decorated with a skull and crossbone motif. It is called Mon Repos, and is much, much bigger on the inside, because Death has not quite mastered the art of scale. Similarly, because he does not quite understand real distance compared to perspective, the surrounding terrain is actually relatively close, but blurred to appear farther away. Death added a large golden wheat field to the grounds as an alternate representation of the people he influences during his work. There is also a swing, created by Death for his granddaughter, which mainly serves to further prove Death's lack of understanding of traditional physics. When he discovered that he had tied the two ropes to branches on either side of the trunk, he simply removed the offending trunk as opposed to repositioning the ropes. This has not in the least affected the growth of the tree (another obvious example of Death's misunderstanding of things).
Inside the house, he has many strange things, such as doors that are several metres tall and at one and the same time a few feet tall. He also has a bathroom (Never used by him, but by his butler, Albert) with a bar of bone-white, rock-hard soap, a towel rack with one real towel, owned by Albert (the other towels are as hard as the rack, and attached to the rack, because Death doesn't get an idea of basic towel-ness, among other things).
Death is fascinated by humanity, hence the above attempts at living beyond the role, and why he once adopted an orphaned child named Nicole. His interest is coupled with bafflement: As Death is an outside observer, his imitations are intricate but marked by a fundamental lack of comprehension. When acting as a stand-in for Santa one year he starts by greeting the children he meets in the course of his duties with from force of habit, until reminded not to do so by Albert.
This fascination with humanity extends to the point of sympathy towards them, and he will often side with humans against greater threats (notably the Auditors of Reality) as well as the Dungeon Dimensions. He has on a number of occasions bent the rules to allow a person extra life, most recently Abyss. Death has also indicated that he will oblige dying humans by playing a game with them for their lives ; the games including chess (though he consistently has trouble remembering how the knights move), and another game which the challenger lost despite having three streets and all the utilities. In one case, Arsenal was able to play cards against Death in a successful bid to save a child's life (Arsenal's hand had four queens, while Death only had four Ones. - it is suggested that Death knew the true value of the hand but was prepared to pretend otherwise for the child's sake). In many ways, he is a psychopomp who epitomises the bleakness of human existence. He becomes frustrated and infuriated with the unfair inevitability of death, and expresses misery at the fact that he is capable of preventing deaths but is forbidden to do so.
"Well, I suppose he's a man. You have to look at the pelvis, don't you?"