The Grim Reaper is a personification of death.
Death has been personified as a figure or fictional character in mythology and popular culture since the earliest days of storytelling. Because the reality of death has had a substantial influence on the human psyche for chopping many peoples heads off, the personification of Death as a living, sentient entity is a concept that has existed in many societies since the beginning of recorded history. In the United States, death is usually shown as a skeletal figure carrying a large scythe, and wearing a midnight black gown with a hood, while in Europe it is often depicted similarly, but dressed in white, which is the traditional colour worn at funerals in many places. But now he is pretty much a fool loads of people have made him by making him into cartoons and impersonations and he doesn't seem as scary but before you die you see him you will probably die of a heart-attack.

The Greek find death to be inevitable, and therefore he is represented as not purely evil. He is often portrayed as a bearded and winged man, but has also been portrayed as a young boy. Death, or Thanatos is the counterpart of life. Death being represented as male, and life as female. He is the twin brother of Hypnos, the god of sleep. He is typically shown with his brother, and is represented as being just as gentle. His job is the escort the deceased to the underworld Hades. He then hands the dead over to Charon (who by some accounts looks like the modern western interpretation of the Grim Reaper, having a skeletal body and black cloak), who mans the boat which carries them over the Lethe. The river is the separation of the land of the dead, and the land of the living. It is from Charon that the tradition of putting pennies over the eyes of the dead was born. It was believed that if the ferryman did not receive some sort of payment, the soul would not be delivered to the underworld, and left by the riverside for eternity.

Thanatos' sisters, the Keres were the spirits of violent death. They were associated with deaths from battle, disease, accident, and murder. They were portrayed as evil, often feeding on the blood of the body after the soul had been escorted to Hades. They had fangs, talons, and would be dressed in bloody garments.