Ryouga is one of Yuuma's close friends and rivals. After being disqualified from the National Duel Circuit, Ryouga became bitter and uncaring, the way he held himself at the beginning of the series. Cold and malicious, Ryouga flaunts his power by taking the Decks of the people he defeats. Once he's defeated by Yuuma in the second episode, and the 'numbers' possessing him is taken, he returns to his more humble self.
Ryouga holds an intense hatred towards IV for setting him up to be disqualified and implying that he intentionally hurting his younger sister. After learning this from IV himself, he entered World Duel Carnival and became more cold and more brutal to any duelist he challenges, even considering Yuuma his enemy.
(Source: Yu-Gi-Oh Wikia)
It was later revealed that Shark is king of the Barian World, Nasch.
Originally born as Nasch, he was once the ruler of the United Lands of the Poseidon Ocean alongside his sister, Merag and was well-loved by his people. One day, his kingdom was attacked by Vector, who led a fleet of ships and several "Gorgonic Guardians" to lay waste to it and loot it. Nasch ordered a counterattack, arming his soldiers with mirror shields which turned the gorgons' abilities against them. Vector performed a blood sacrifice in response, calling forth the ocean god, Abyss, turning the tide of the battle again. Merag rode up to Abyss and was grabbed by him. Despite the intervention of Nasch's friend, a knight named Durbe, Merag did not want to be saved and sacrificed herself to call forth another god to purify Abyss.
After Merag's funeral, Nasch declared an attack on Vector's kingdom in retribution, as the latter has fled with the god turned to Nasch's side. Once they got to a town ravaged by Vector's army, Nasch noticed a little girl crying, who strongly resembled Merag. Nasch took the young girl, Iris, under his care and wondered if she was his deceased sister's reincarnation or if she was sent by Merag herself. As Nasch and his army pursued Vector, Nasch grew close to Iris and planned to confront Vector by himself once the battle started. On war front, Nasch followed Vector to a secret tunnel in what would eventually become a Number ruins, where Vector challenged him to a Game of Darkness. Per the Duel's rules, the more monsters of either ruler that were sent to the Graveyard, the more soldiers their army would lose. Though Nasch was victorious, his entire army was lost due to Vector's mill strategy. Their souls were sent to the Barian World. Vector barely survived and limped back to his own country, prompting Nasch to follow him. There, he found Vector had killed all of his own people. He also witnessed Vector being dragged to the afterlife by the people he murdered
An unknown number of years later, Don Thousand reincarnated both he and his sister as two of the Seven Barian Emperors, as their souls were each connected to a Legendary "Number" card, which had sealed Thousand's power. Nasch then became the leader of the Seven Barian Emperors, much to Vector's disdain (also having been reborn as a Barian Emperor). After Nasch had annoyed him enough to score 100,000,000 points, Vector enacted his revenge by taking Merag hostage. When Nasch intervened, Vector used "Giant Sky Sword" to open a bottomless rift. He then pushed Merag from the cliff, forcing Nasch to follow after her only for the two to be sucked into the rift. The two were rescued by Abyss, who informed them that he would send them to the Human World without their memories, where their souls were then revived in the bodies of Kamishiro Ryouga and Rio, who had recently died in a car accident. While Vector believed them dead, Durbe believes they had simply disappeared and assumed leadership of the group from them in the interim, waiting for their return.