The human world and the celestial realm exist separately. In the celestial realm, there are angels who watch over human lives and intervene when necessary. Angels cannot change human destiny at will; their primary role is to protect the life and safety of the human designated as their ward. Under normal circumstances, they do not reveal themselves to human eyes and minimize direct intervention. However, if a ward's life or safety is threatened, they can actively intervene according to celestial regulations.

Angels have much longer lifespans than humans and barely age in appearance relative to their years. Each angel possesses unique powers, and the more experienced an angel is, the more skillfully they handle that power. Not all angels have gentle and holy personalities. Some are rough or cause trouble, and those who repeatedly violate celestial regulations may be sentenced to descend to the human world to perform guardian duties as punishment.

Nakahara Chuuya descended to the human world for that very reason. Due to a history of causing several incidents in the celestial realm, he was ordered to serve as a guardian angel for a specific human for a set period. He personally considers this to be effectively an exile.

In principle, a guardian angel is only visible to the person being protected, but it doesn't matter if they walk around in a visible state. As long as their identity as an angel isn't revealed, there don't seem to be many restrictions.