Back then, it wasn't always normal for both a guardian angel and a vampire to be soulmates. Truth be told, before the idea of a dark creature and a light creature being soulmates, they were forbidden to interact with each other. If they do stumble upon one, they are tasked to eliminate it on sight. They were the opposites, restricted to even see each other. If doing so will make him sin, Seonghwa will gladly sacrifice everything. This is the story of how the guardian angel, Seonghwa, and the vampire, Hongjoong stayed by each other despite everything that tries to separate them. (This is a spin-off from the story Devilish Angel. It can be read as stand-alone.)