Rui is the king of the Demon relm. The Demon relm is great enemies with the Human relm, especially the Wonderland Kingdom. Rui always had battles with the head knight of the Wonderland Kingdom. The head knight was named Tsukasa. Even though they fought, Rui had started feeling bad for Tsukasa as the years went by and they bacame friends and their battles became training sessions. Currently, the two were training.