I see two possibilities (not that there is only two). First they were enemies in battle, even though they were childhood friends, and when they fight he is victorious. But he still loved her.
Second, he is the powerful dak king who kidnaps the light king's daughter. During the time they spend she sees a different, kind side to him and falls in love. When her father and him go into battle she doesn't want either to get hurt so she runs to stop them. He listens but her father doesn't and tries to kill him, but she blocks the spear and saves him.
