Riou, (リオウ, Riō) also called Leo in some manga translations, and Banikis Gigo (バニキス・ギーゴー, Banikisu Gīgō) are the main antagonists of the first half of the Faudo Arc. Riou's partner goes unseen for most of the arc, but was shown to have been hiding in the mouth on Riou's chest. Riou utilizes Beast Magic, which is similar to some of the final spells of many mamodo that are in the forms of animals. In the manga, Riou placed 26th in the battle overall.
Before he left for the human world, his family has been protecting Faudo for ages until Riou's father places a timer on it to appear in the human world. Riou was told by his father to use Faudo to his advantage in winning the kingship and provided him with a jewel that would let him control Faudo, Riou had a very abusive father who often beat him because he thought Riou wasn't capable of being king with his own power, so Riou's father forced him and tortured him to learn "the curse" and also insulted him because he was considered weak, then Riou tried his best to revive Faudo not specially to become king, but to make his father proud of him and possibly, his father would see him in another way.
After a while in the human world, he goes to a mountain range where Faudo is teleported to. He starts to assemble a group of like-minded Mamodo and using a death curse to bend good Mamodo to his will, provide him with all the "Dioga" level spells he needed to release Faudo.
Being set back from the loss of Buzarai, Riou needs a new person to replace him. He is unable to find anyone but knows that Zatch will come to save his friends and break the seal. As soon as the seal is broken, he goes into the control room, knowing the others will come after him. he demonstrates Faudo's power by having him fire at the ocean, creating a huge explosion. He offers to spare those who become his servants, gaining most of the mamodo that helped him break the seal. He then orders Faudo to eliminate Kiyo and the others but is confused when Faudo misses all of them (due to Kiyo messing around with Faudo's system). He realizes it but is too late and Faudo is teleported nearby. He laughs since it didn't take him far at all. But he gets shocked when Faudo starts sinking (due to being on the Kermadec Trench), but is relieved when he finds out Faudo can swim. He interrupts Kiyo and the others' celebration and tells Kiyo that for all he has done, Japan will be Faudo's first target.He and Banikis are both responsible for Kiyo's health (meaning his heart that gave him anxiety) and death (manga only).
Riou's role in the story changes from the anime and manga.
Riou announces that he has turned off the power to supply the device which will transport Faudo back to the Mamodo world. In the control room, he oversees all the battle of his minions and gives them support from Faudo's support system, only for all of them to still fail. When
Arth and Ellie reach the room before the control room, they are confronted by Riou. Riou uses a nearby control panel and seals the entrance to the room off so the rest of the team are not able to assist him. Arth gains the advantage against Riou and severely injures him. He reveals that using the devices in Faudo, he is able to restore his health completely. Arth rushes through Riou's attacks and using the control panel, opens the entrances to the room. At that time, Riou's Fanon Riou Diou defeats Arth and burns his book. The rest of the team enter, they are able to keep up with Riou's attacks. Riou tells his book owner Banikis to come out and fight. Banikis is revealed to have been hiding inside Riou and once out, and tells them that the timing to cast spells will be easier now and overpowers the team. Zeno and Dufort arrive shortly after and prepare to battle Riou. Zeno completely overwhelms Riou with his weakest spells. Banikis uses Girfadom Barsuruk to transform Riou into his "beserker" form but Zeno was still able to overpower him and uses Jauro Zakeruga to finish him off, and subsequently takes the command of Faudo.
As soon as Zatch and his group infiltrate Faudo, Riou appears and separates Zatch and Kiyo from their group. He asks Kiyo to undo the encryption key to Faudo's system, but Kiyo refuses. He calls Banikis to come out of his chest and join the fight. He uses Fanon Riou Diou but Zatch is able to counterattack with Rauzaruk. Banikis shows them Faudo Liquid, which can recover his health and heart energy. They are able to overpower Zatch and Kiyo with their recovery, and use Vargas Farlon to attack Kiyo. Riou decides to attack all of Kiyo's body parts one at a time when Kiyo tells Zatch not to protect him. While dodging Zatch's Zagurzems, each attack they send incapacitate Kiyo more to the point of almost killing him. While attacking Kiyo, Zatch manages to him with Zagurzem. Kiyo casts Bao Zakeruga as his last act before dying and connects all the Zagurzems to attack Riou and deal heavy damage. However, Riou survives and sends Banikis to burn Zatch's book. As he gets near the book they are interrupted by Wonrei and the others and chose to retreat to recover.
After quickly recovering, he goes into the main control room with his partner where they encounter Zeno. They start to fight but Riou is easily overpowered by Zeno. He calls for help to the people in Faudo, but Zeno says he won't be around when they come to his aid. Banikis uses Girfadom Barsuruk to transform Riou into his "berserker" form but Zeno was still able to overpower him and uses Jauro Zakeruga to finish him off, and subsequently, takes the command of Faudo. Soon Zeno puts Banikis in one of the pillars in the control room but was able to get out with help and evacuated from Faudo after Zeno's defeat.
Riou also has the ability to curse humans, causing them to become weak and eventually die. Riou's will is the only thing that can break the curse. The curse has been passed down through his family for ages, for reasons unknown. Although, using this spell also puts a strain on his own body, especially when used on many people. He uses this ability to receive help on breaking Faudo's seal by making a few Mamodo bookkeepers help him to free Faudo, or die if they didn't. The humans that were affected by this curse were Nicole, Ellie, Alishie, and Li-en. However after cursing these four, Riou acknowledged his limit and remarked that he would die if he used the family curse again.