Angels and demons. Two races that have been in conflict since time immemorial. Among the demons was a famous playboy named Zeil, whose hobby was corrupting angels and making them fall from grace. However, he was the one who ended up falling for the angel Guest, and in a twist of fate, he became an angel himself!

Angels Guardian deities who maintain order in the human and celestial realms. They praise God and act as messengers of the divine. They frequently descend to the human realm for work. (They fly through the sky.) All angels have light hair colors (gold, beige, light brown, white), white wings, and a glowing yellow halo floating above their heads.

Demons Beings that feed on human desires and emotions. They interfere with human desires and stir up their feelings. Their job is to descend to the human realm to draw out people's emotions and instill desire. (Like angels, they fly through the sky.) All demons have glossy black hair, two short, hard black horns, and black wings that contrast with those of angels.

An angel being corrupted into a demon is a grave taboo in the celestial realm. A demon losing their heart to an angel is considered a "defeat" in the demon realm. The wings of a fallen angel turn black, their halo becomes dull and dark, emitting only a faint light. Eventually, the halo disappears, horns grow, and they become a demon. It starts with black feathers appearing here and there on the wings. Next, the halo's light weakens, by which time the wings are about half black. Conversely, a defeated demon's glossy black hair turns gray or gold (though it never becomes as bright and beautiful as a true angel's), and their wings change to pure white. Their horns vanish, and an angelic halo appears. It begins with a few strands of light-colored hair. Eventually, white feathers mix into the wings, by which time the hair has turned gray.

Neither falling nor defeat can be noticed by the individual at first. They become aware through physical changes. Even if they don't want to believe it, they cannot resist the transformation of their body. Once the change begins, they have no choice but to accept it.