Ryuo Sukuna (龍王・宿儺) is the Dragon Lord of the seven seas, who had a been waiting to seek revenge against Tomoe for 526 years for ripping out his right eye and attacking the north gate of Ryūgū Castle. He found Tomoe whom had just saved a girl from drowning in the sea. The Dragon Lord's eyes are known as the Longevity Elixir and are said to give great power to whoever has drank them. Though aggressive and temperamental, he is afraid of his wife's anger. She disapproves of his fights, saying they mar his beautiful face. He is forced to give up his grudge against Tomoe after she befriends Nanami.
When Nishiki attacked Kotarou and Himemiko in her human form, mistaking her for a regular human, the Dragon King came to Himemiko's defense. After Kotarou was brought to the hospital, he stayed with him, even after Himemiko had gone out to find Kotarou's rubix cube. He told him that Himemiko was a yokai, the Princess of the Swamp. Unable to believe that Himemiko had hid this from him all this time, he tells Himemiko to go away when she comes back with the cube. After he was released from the hospital, he sits outside with Ryuo. Ryuo asks him if this was the way he wanted to end their relationship, with the harsh words he'd said and that if he wanted to say anything he should say it now and he would pass it on to Himemiko. When Kotarou doesn't reply, he tells him he is troublesome. If he wanted Himemiko to have told him, he should be a reliable man, like himself. Kotarou immediately knows he's right but he'd already disappeared. He looks for Himemiko's swamp and Ryuo appears and helps him finally apologize to Himemiko.
Ryou is a proud person and he likes a good fight. He knows how to be a good reliable man although his like for violence is something his wife disagrees with. He always keeps in check what people owe him and can keep a grudge for very long periods of time.
His wife whom he loves very much but is afraid of her at the same time. She often tells him to stop his senseless fights but he often forgets and ends up suffering her wrath.
He held a grudge for him for 526 years ever since he ransacked his castle and stole his right eye. However, after his encounter with Nanami, a little blackmailing from Mizuki, and fear of his wife, he finally let go of his grudge on Tomoe.