The heroine believes that the two are both childhood sweethearts and happy enemies, but she never notices when their friendship turns sour.
The male protagonist is an angel who was put into the human world for training by God. Because he was too cold, arrogant and withdrawn, and because he lacked warmth since childhood, he was cold to the people around him, and a smile never appeared on his face. God wanted to see his smile, so he put him with her. Because she is full of warmth.