He is cynical. He wants to be a hero not because he has a heart to save the world, but because he is looking for his father's footprints.
She is simply without any scheming. She was raised by wild beasts and knew nothing about the world. Even if she wholeheartedly pursues the elder brother who loves her, it is just an instinct to seek warmth.
When he met her, he couldn't let her go or leave her. Spend the last days of this world together...
When she met him, she couldn't leave him or forget him. The gears of fate turn until the end...