He and she are not from the same world at all
He was a human being who once belonged to a wealthy family. Later, his family fell into ruin because he was deceived by a traitor, and he lived in a foreign land. He started from scratch, severely damaged the wealthy family, and vowed revenge.
She is the descendant of demons and gods, with the ability to destroy the world. Her birth itself was a mistake. She was sealed in the underworld of Hades since she was a child, with only unicorns for company. She has no idea of her own abilities and origins, and she can survive without food. Later, during the battle between the demons and gods, a demon broke into the underworld of Hades and released her.
His encounter with her was destined to be a tragedy.