Basically, the Angel is the administrator of the System for Jinwoo and a servant of a dormant cosmic entity called The Shadow King (also translated as Shadow Monarch, but I think Shadow King sounds better). The purpose of the system is to allow Jinwoo to slowly strengthen his body and mind to be able to serve as the vessel of the Shadow King. The Angel's goal is to implant the Shadow King's Black Heart into Jinwoo and have the Shadow King possess him, transforming Jinwoo into the Shadow King's puppet and avatar.
Suffice to say, this will not go quite the way the Angel expects.