She lives in the human race, and it is her nature to save lives and heal the wounded.
He lives in a devil's palace, and his nature is to kill without reward.
She was one of the many gifts chosen by the human race to honor the Demon King.
At first, he was just teasing her out of boredom, but later on, when he discovered her steadfastness, he was willing to spend his life for her just to win her smile.
When he woke up from the edge of death, he found that her beautiful black hair had turned so white.