In the time before Time existed, angel fought angel in the War in Heaven and two lovers parted. One decided to rebel against the Creator, the other to obey divine law.
Some angels turned into devils, corrupted and corrupted. Some decided to stay apart and neutral, indifferent to the War. One of the lovers, the disobedient one, took this path.
The two sides made an uneasy truce. Except for rare circumstances, the angels would not reveal themselves to each other. But the truce has started to unravel. Mortals have begun to see signs and wonders as angels has begun to take arms against one another.
One lover has come to Earth, illegally, having committed an act of treason herself to have done so, to warn her lover and try to give her one last chance.
The premise comes borrowed from, with her permission, an idea from May Be Apples of Eris. Action would shift between the near present day and the rebellion. I would play the rebel angel, perhaps one who has lived for a long time as a human, having forgotten her origins, though we don't have to that if you don't like, likewise with other plot elements I have detailed. Probably would want to get more into epic non-consensual romance, with non-consensual elements if you would like to go in that direction.