Riou's father is a large mamodo and part of a tribe that guards Faudo in the mamodo world. He only appears during Riou's flashback during his fight with Zeon.
Riou's father tortured his own son because he believed Riou was weak and forced him to learn "the curse" and how to control and provided him with a jewel that would let him control Faudo.
Before his son Riou left for the human world, he told him he represented his tribe. He also told Riou to use Faudo to his advantage in winning the kingship when it teleports to the human world.