Death (Discworld)
Death has appeared in every Discworld novel, with the exception of "The Wee Free Men" and "Snuff", and had a possible cameo at the end of "Johnny and the Dead" (the character was not identified, but spoke in unquoted small caps). The Discworld books with Death as a leading character, starting with "Mort" in 1987, are:
Death (Discworld)
Death is not invisible; however, most people's brains refuse to acknowledge him for who he is, unless he insists. Under normal circumstances, only those of a magical disposition (e.g. witches and wizards), children, and cats can see him, or allow themselves to see him. Due to his eternal nature, Death can ignore such things as walls and magic spells; he exists throughout all time, so things lasting merely centuries are not as real as he is. He can adjust time with regard to himself and others near him, completing tasks very rapidly from a normal perspective. He can only go where people believe in death and can only see people who can die. In this sense, he is similar to the gods of the Discworld, in that he is powered by mortals' collective belief in him.