Reapers are the elite servants of The Gate Keeper.
They act as mercenaries that hunt down rebelling Altered and humans that have escaped the Path of Black Leaves before they could be turned into Altered. As such every single one of them is personally trained by the Gate Keeper himself to ensure they are capable of surviving battles with even the most powerful Altered.
All Reapers have extensive knowledge of the Altered with the exception of prototype Altered due to the fact that Slenderman only trusts The Gate Keeper with said information. Unlike the Gate Keeper they do not live in the Path of Black Leaves and live somewhat normal lives in the human world when they are not on missions.