Originally the guardians of the Gates of Hades Valley (the place where the evil souls go), when Hell War IV started, the souls slowly stopped going there. The were called by Abaddon to protect the Demon Monastery because he was going to fight in the war. By the end of the war they where taken prisioners by the morlocks until the Cold War where the Devilsons free them and helped them into rebuilding the Blood Palace.
After being released from the morlocks, the Reapers offered total loyalty to the Devilsons. They signed a contract with the Devilsons, each Devilson member is allowed to have a Reaper as a personal guardian. Only Krinky and Neko are the only ones that shares the same reaper, called Semmek. For a Devilson to receive a Reaper, one most beat the Reaper in a duel, help him in something deeply important or save their lives (this only works on Devilsons).
Recently, a comunity of Reapers live within the Blood Palace walls. They live to assist the Devilsons in any kind of way. They act as most of Devilsons guardians, giving them acces to some of their abilities, only if they deserve it. They may look scary, but Reapers are somehow shy, but loyal to the Devilsons, they would sacrifice their lives for them. Another group of them moved back to Hade Valley, but they still respect the Devilsons. Those who work on the Hades Valley are called Hades Reapers or Soul Reapers by others, the ones that works under the Devilsons domain are called Devil Reapers.