Reapers

Reaper are sinners that are chosen by Satan to act as his executor. Once a sinner has died, if Satan choose to, he offer them to become a reaper or going trough their sentence. If the sinner accept the deal, he will give them a new undying(but not invincible) body, a outfit, and will stitch a demon to their soul to grant them their reaper power.

They are moslty tasked to kill one sinner per day, marked by a red X. They can do other task if satan choose to. If the reaper miss his sinner quota, it will be sent to his sentence in hell. However, if Satan want to, he can still give another chance to the reaper(Brooks had missed his quota and was dismembered by a demonic Scarlet, but Satan still repaired him and gave him another day, and he implies that he missed his quota multiple time in the past). Some reaper have a harder quotas, as Ashe, the most powerful reaper of Satan, has to kill 10 sinners per days.

They are special circunstance for one to become a reaper. Altought it's not specified clearly what would be such circunstance, in the case of Brooks, he became a reaper(altought he was not a sinner) to avoid eternal punishment to his sister, wich explain the lack of sinner mark on his body.

According to Miss Celeste, Satan is not allowed to have more than three reapers, as there is three archangel, revealing the fact that archangels and reapers must be in even number. Scarlet is apparently the fourth, but Satan tells that she is a replacement for a reaper called Wyrnn, as she is apparently gone.

Regardless of their control over their demons, Reaper can have various degrees of innate power, and Scarlet was noted to be particulary powerful, enough at least to damage Satan body. Ashe, a unknown reaper, is noted by Satan to be his most powerful reaper "by almost double".

Powers
Grim reaper physiology: their body are mere "puppet" of flesh made by Satan for their soul to inhabit and animate. Thus, they are imprevious to age, and they remain functional and "alive" even if dismembered(Brook was still "alive" and fully awake while he was decapited). Altought it was only seen on Scarlet so far, reaper possess little horns on their skull, and have unatural color in their iris, as well as a demon tail. In the case of Brooks, he was confirmed by GRAVEWEAVER to have lost them long time ago(the reason why or how is unclear).

Demon empowerment: Reaper are bound to a demon stiched to their soul, wich grant them hellish power that stem from it. However, if they become too emotionally unstable or draw too much of its power, the demon will take over and cause havock around him. Skilled reaper are far more proeficent, and can use great lenght of it's power without much problem.

Super strengh: the demon inside a reaper passively increase their strenght. Scarlet was able to kill sinners without any problem, and break handcuff without much effort. Reapers such as brooks can actively draw more power by envelopping their fists with demon energy, and can even cause serious damage to very hard material such as concret.

Weapon conjuration: A reaper can conjure a massive scyth and use it to provoke massive damage. It's unknown if this weapon is unique to Scarlet, or if reaper can choose what weapon will appear. It's the most "simple" power, requiring absolute no training and does not ask much power from the demon.

clairvoyance: A reaper can identify sinners by the big red X that marks them. Brooks told Satan that he could "smell" the red demon within scarlet.

Known Reaper

 * Scarlet (currently in the nine circle, used as a "replacement" for Wrynn)
 * Brooks
 * Wrynn (status unclear, quoted to be "gone")
 * Ashe