Reapers

Reapers are servants of Death. They assist in maintaining the Natural Order, and are completely neutral, not aligned with Heaven or Hell nor Purgatory. According to Castiel, Reapers will sometimes appear in mass gatherings in prelude to a great disaster such as the the eruption of Pompeii.

Background
Reapers are first credited in John Winchester's journal. Dean and Sam Winchester first encounter a Reaper in the episode "Faith", seen assisting Roy Legrange "heal" the sick. However, only someone who is dead or near-death can see a Reaper. The Reaper can be controlled by Black Magic and was released by smashing the Rosario controlling it, upon which it turned on the person who was controlling it: Roy Legrange's wife.

A different Reaper, Tessa, returns in the season 2 opener "In My Time of Dying". After Dean is left in a near-death coma following a car accident, the Reaper, disguised as a young girl, eventually persuades Dean to come with her. However, John Winchester offered his soul and the Colt to restore his fallen son. The Yellow-Eyed Demon, then taking the offer, possesses Tessa, allowing Dean to live.

Tessa has a second appearance in the fourth-season episode "Death Takes a Holiday", along with another reaper, in which Sam and Dean to try and stop the breaking of a seal, which involves two reapers being killed.

Reapers make a fourth appearance in the fifth-season episode, "Abandon All Hope...". Hundreds appear in a deserted street through Castiel's eyes. Bobby later reveals that they are in the town because Lucifer is planning a ritual and raising their "Boss" Death, one of the four horsemen.

Reapers make a 5th appearance in the fifth-season episode, "Two Minutes to Midnight". They appear through Crowley's eyes, standing outside of a building where Death supposedly is.

Tessa makes an appearance in the 6th-season episode, "Appointment In Samarra" along with the Horseman Death. This marks the 6th appearance of reapers on the show.

Reapers make a 7th appearance in "Death's Door" when a reaper comes after Bobby in his coma when he is shot by Richard Roman.

Powers

 * Invisibility - Reapers are naturally invisible to the naked-eye, unless they're dealing with the deceased, or if they chose otherwise. Angels like Castiel are capable of bypassing this skill and see reapers with clarity.
 * Teleportation - Reapers can just appear anywhere they choose.
 * Resurrection - Reapers are capable of reviving the dead, though they generally don't.
 * Terrakinesis - Reapers can shake rooms just by being present.
 * Time manipulation -
 * Astral projection -

Family

 * Death (Father)
 * Other Reapers (Kindred)

Season 1

 * "Faith"

Season 2

 * "In My Time of Dying"

Season 4

 * "Death Takes a Holiday"

Season 5

 * "Abandon All Hope..."
 * "Two Minutes to Midnight"

Season 6

 * "Appointment In Samarra"

Season 7

 * "Death's Door"