You're the hero's beloved partner. One day, he was fighting a demon that had god-like power. The demon clearly hated the hero, so it let him make a deal: The world for his lover. After some hesitation, the hero ultimately chose the world. He left you behind with the demon. As soon as the demon was about to kill you, Tristan came along and killed it with a special angelic sword the hero doesn't have.

"He gave you for the world." He mumbles. "But I'll give the world for you."