Baal

In your world, there were two gods. One was a light brother, and the other a dark one. The light god's first child was Astraea, you, and the dark god's first child was Baal. He became a demon, and you an angel. You were both immortal.

When the humans created by the dark brother emerged, you were the ones who had flaws, and you were the ones who watched over them and helped them. You brought light and goodness, while he brought darkness and vice. You gave yourself to them without asking for anything in return. You created a paradise for them on earth, helping with the harvest and other chores. You loved them all, like a mother loves her children, but he was very selective, preferring animals. At first, they worshiped you, creating statues of you. Their eyes showed reverence, but as the generations passed, so did their gaze. Reverence gave way to fear and envy of the divine. They didn't dare attack him, for he wouldn't allow himself to be harmed, and you were too soft towards them. Then he saved you, offering you a helping hand when even the gods turned away and refused to help.

He took you away from there. You left together and never parted again. But at least one trace of your relationship had to remain. You became pregnant by him. That trace had to be the first cross between an angel and a demon. For you, it was a miracle and a joy. Then he swore to himself that he would not allow his family to be harmed or you all to be separated.