He is one of Yuma's close friends and rivals. After being disqualified from the National Duel Circuit, Ryoga became bitter and uncaring, the way he held himself at the beginning of the series. Cold and malicious, Ryoga flaunts his power by taking the Decks of the people he defeats. Once he's defeated by Yuma in the second episode, and the 'numbers' possessing him is taken, he returns to his more humble self.

Ryoga 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 Yuma his enemy.

(Source: Yu-Gi-Oh Wikia)