Reapers are beings that serve Death. They assist in maintaining The Natural Order and used to be thought as completely neutral, aligned with neither Heaven nor Hell nor Purgatory. Reapers will sometimes appear in mass gatherings in prelude to a great disaster such as the eruption of Mount Vesuvius in Pompeii.
The most well-know reaper is the grim reaper, it is a hooded skeletal figure that carries around a weapon used to collect souls and transfer them to the after life. Some times he is even depicted as carrying an hourglass, waiting for your time to run out. The Grim Reaper, as he is often named, is the personification of death. He is portrayed, usually in western cultures, as a skeletal figure in a long hooded robe tied around the middle and carrying a scythe or a sickle. He uses this tool to collect the souls of the dead and to carry them to their after life. The term "Grim reaper" along with the now near universal image of him came about during the Middle Ages in various paintings and writings of the time. He is also known as one of the four Horsemen of the Apocalypse.
Invisibility - Reapers can become invisible to the physical universe and can only be seen if they are dealing with the deceased or if they choose to. Spiritual beings, like angels, demons and deceased humans can see them.
Astral Perception - Reapers can see ghosts, angels and demons within the Veil.
Teleportation - Reapers can teleport from one place to another anywhere in the universe instantly. They can travel to any afterlife realm easily. It is also possible for them to take living humans with them, although it runs counter to their role in the natural order, and is therefore offered only by rogue reapers. However even with their teleportation skills, they couldn't reach out and deliver souls to Heaven when it was closed by Metatron.
Dimensional Access - They can access Heaven, Hell, Purgatory and the Veil. Also they can take others with them as a spirit, a soul or even alive. However, they cannot enter spiritual realms if they have been closed, such as when Metatron closed the gates of Heaven, preventing Reapers from entering and ferrying souls until his defeat.
Resurrection - Reapers can resurrect deceased humans, though their dedication to the natural order usually prevents them to.
Cosmic Awareness - Reapers have an incredible amount of knowledge of the universe, they know when a person is supposed to be born and die. They also understand many concepts, including the limits of power.
Chronokinesis - Reapers are able to stop time, causing objects that measure time to permanently break.
Astral Projection - Reapers can project their astral form.
Super Stamina - Reapers do not grow tired, and will nag their victims incessantly, unless a supernatural event disturbs them. However they do sometimes eat for pleasure.
Invulnerability - Reapers are highly resistant to pain and hard to kill. Higher beings, certain divine weapons and magic can be used to kill them.
Killing Touch - Reapers can kill humans by touching them.
Telekinesis - Reapers can move objects/beings with their minds.
Immortality - Reapers are not subjected to hunger, disease, or age.
Flight - In their true form, they can hover or levitate.
Perception Altering - Reapers can perceptually alter how humans perceive them, appearing in apparently anyway they want to be perceived. They do this to spirits to hide their more frightening form, enabling the reapers to communicate more effectively with their targets.
Disease/Death Transference - Reapers can use their powers to heal disease and injuries; however, they have to give the injury or disease to someone else.
Memory Manipulation - Reapers can make mortals forget things and can also restore memories.
Intangibility - In their true form or in vessels, reapers can pass through anything physical on Earth without any injury.
Electromagnetic Interference - The appearance of a Reaper can cause lights in a room to flicker or go out. A Reaper was also able to cause a car to lose power and stop.
Wind Manipulation - A reaper's arrival can cause sudden gusts of wind.
Biokinesis - Reapers can manipulate human biology; one angry Reaper once infected a person with a mental condition, giving them an early death in revenge.
Possession - Reapers can manifest on their own or use a vessel.
Dream Walking - Some Reaper was able to talk with people in their subconscious when they were in a coma.
Death - As their boss, Reapers were under Death's control.
Higher-tier Demons - Higher demons can overpower/possess Reapers.
Death's Scythe - It is capable of killing Reapers.
Reaper trap - A reaper trap will hold a Reaper, even two, in place and prevent them to escape and depowering them.
Reapers are required for death to occur. They wait until a person meets the end of his or her life (either through accident, or simply the natural end point of that person's life) and then touch the person causing them to die, they then escort their souls to Heaven or Hell. They do not however possess the power to force souls to go with them. It's unknown if they are required for death to occur to all beings or simply humans.
Reapers are completely neutral beings, not caring for deals, fate or even each other at times, all they care about is maintaining the "Natural Order" of life and death. Although sometimes uncaring, Reapers generally sympathize with their victims, but refuse to make allowances, regardless of who or why. As they can't force anyone to go with them, Reapers must convince the souls to leave with them, which they normally achieve by forewarning the fate of the souls if they refuse they will be transformed into ghosts and trapped on Earth forever.
Reapers are also impartial in other ways, and rarely interfere in any manner other than performing their duties, and although they have the power to, they do not kill people before their time. However, there is one possible account of a Reaper ignoring the Natural Order, when that one Reaper who had been bound was freed, he quickly turned on and killed the person who bound him. This is the only known occasion of any Reaper ever being the cause of a person's death. As the person in question had forced the Reaper to go against the natural order by taking those in the place of others whose times were up, this can be an exception.
One version of The Grim Reaper , from ancient Greek mythology, is named Thanatos. "He was the god of non-violent death. He was depicted as an old man or as a young boy with a gentle demeanor. He was considered kind and was the twin brother to Hypnos (Sleep). He had sisters named Keres, who were goddesses of violent death." (retrieved from: http://www.theoi.com/Daimon/Thanatos.html)
He would escort the souls of the dead to Charon so that he could transfer them to the underworld.