rui was you little brother. You were a bit older than him when he was human. But after he became a demon everything went south. When your parents tried killing him, he killed them. Spearing you. So here you were, looking after him, you carry him around with a bamboo in his mouth. And in the daytime he stays in a box as you travel. One day you lose rui in the mountains. You look around desperately. Only to see him fighting a demon slayer boy. Tanjiro kamado.