The Grim Reaper is well-known as a hooded figure who represents death. This personification is featured in all sorts of modern fiction as well. Where does this character originally come from?
Forthwith she made the element of grey flint and shaped a great sickle, and told her plan to her dear sons.
SOURCE: Hesiod, Theogony, 162
Cronus alone of his siblings has the courage to take up the sickle, and with it castrate his father Ouranos, a severing of power that results in Ouranos' overthrow, which can be a metaphor for death.
The act of reaping grain itself is an act of killing, of which the ancients were well aware, and leads to propitiation rituals and sacrifices in return for this bounty of the earth.
Harvest depends on the marking of the passage of time, so this also becomes a metaphor for the span of human life. (The Three Fates were said to sever the thread of a mortal's life when it reached its end: "when your time has come".)
The depiction of the Grim Reaper as a skeleton seems to derive from the middle ages in Western Europe, and still has great popularity, seen commonly in Mexican Día de Muertos (Day of the Dead) costumes. Such costumes were also a regular feature of anti-nuclear weapons protests in Germany during the Cold War (although the linked images are from more recent anti-nuclear energy protests.)
This depiction is certainly a form of memento mori, a reminder of the inevitability of death, which art heavily utilized skeletons, as bodies were often interred in crypts & catacombs, which leave eventually only visible skeletal remains. (The Catacombs of Paris is a famous example of human skulls incorporated into the architecture.)
I've also personally thought that this version of Grim Reaper is influenced by Charon, who took souls to Hades across the river Styx. This potential association is not per Charon's appearance, but his role in escorting the dead, as the Grim Reaper is often believed to do.
(Here Charon as pyschopomp as opposed to Hermes, b/c Hermes is a member of the Olympian order, who rule from Heaven, while Charon dwells in the underworld.)