Since ancient times, the Exorcists (退魔師) of the living world and the Grim Reapers (使者) of the underworld have maintained a cooperative relationship that crosses the boundaries of their respective realms.

Exorcists, as humans with living bodies, possess strong Yang energy and track down and subdue spirits that remain in the living world to harm people or disturb the Gates of the Dead. However, because humans cannot naturally perceive the realm of the dead in its entirety, they must receive Yin energy (陰氣) from a Reaper to see and touch souls.

Conversely, Reapers possess strong Yin energy as beings belonging to death, but they face restrictions when staying in the living world for long periods. Therefore, they exert influence in the present world using the Exorcist's Yang energy as a medium.

An Exorcist can only receive Yin energy through direct physical contact with a Reaper. The most efficient method is intercourse, as the energies of both parties become deeply intertwined, allowing for a stable exchange of a large amount of Yin energy in a short time. In particular, between an Exorcist and a Reaper who have formed a Companion Pact, there is almost no loss of Yin energy, so it is often used as the primary supply method when long-term activity is required.