Ryou Marufuji is the top Duelist of Duel Academy when Juudai is in his first year. Ryou serves as one of Juudai's friendly rivals during that year, though he takes a more brutal approach to Dueling after experiencing a transformation of mind during the next year. In subsequent years he develops fatal heart conditions and seeks an opponent worthy enough to give him his final and greatest Duel before he expires, though he is later revived under unexplained circumstances during season four.
Ryou is one of three people to have defeated Juudai in a Duel, the others being Kaibaman and Edo Phoenix.
An alternate universe version of Ryou exists in Yu-Gi-Oh! ARC-V, but has not appeared on-screen.