Duke Devlin, known as Ryuji Otogi in the original Japanese version, is the charismatic creator of the dice-and-board game Dungeon Dice Monsters. In the anime, he is the proprietor of the Black Clown game shop, a rival establishment to the Kame Game shop owned by Yugi Muto's grandfather. His birthday is February 28, and he is known for his confident and flashy demeanor.

Initially, Duke is introduced as an antagonist driven by a personal vendetta. His primary motivation stems from the disappearance of Maximillion Pegasus, the creator of Duel Monsters. Duke had previously demonstrated his game to Pegasus and was impressed by the man's skill. After Pegasus was defeated by Yugi at the Duelist Kingdom tournament, Duke refused to believe a fair player could have won and blamed Yugi for Pegasus's vanishing. This belief, combined with a desire for revenge and a challenge to Yugi's title as the King of Games, fuels his early antagonism. In the original manga, this rivalry has deeper roots, as Duke's father once lost a dark game to Yugi's grandfather, Solomon Muto, which aged him fifty years in a single night, creating a family legacy of revenge.

Duke's personality is defined by a love for theatrics, a sharp competitive streak, and an unshakable confidence in his gaming abilities. Upon transferring to Domino High School, he quickly becomes popular, impressing classmates with flashy dice tricks, which immediately stirs jealousy in Joey Wheeler. He can be arrogant and ruthless, demonstrated when he publicly humiliates a defeated Joey by forcing him to wear a dog suit as a penalty for losing a Duel Monsters game. Despite this harsh exterior, Duke possesses a deep respect for the integrity of a fair game. After losing to Yugi in a match of Dungeon Dice Monsters, he comes to realize that Yugi won honestly and did not cheat Pegasus. This defeat, coupled with the compassion shown by Yugi and his friends, prompts a significant shift in his character, transforming him from a bitter rival into a loyal and kind-hearted ally.

Following his initial defeat, Duke's role in the story transitions from an adversary to a supporting friend of Yugi's group. He and his game shop, the Black Clown, become integrated into the circle, often accompanying the main characters on their adventures. During the Battle City tournament, he helps rescue Serenity Wheeler and Tristan Taylor from the Rare Hunters, showcasing his bravery. He develops a noticeable crush on Serenity, which creates a friendly but competitive dynamic with Tristan, who shares similar feelings. He later participates in the Virtual World arc, where he teams up with Tristan and Serenity in a crucial duel against Nezbitt, a member of the Big Five. Duke's strategic mind and expertise with dice prove vital to their victory. Throughout these later story arcs, he often serves as a commentator and a source of support for his friends, having fully left his role as an antagonist behind.

Duke's most notable contribution to the series is his creation, Dungeon Dice Monsters, a complex strategic game that combines elements of role-playing and board games. His expertise in this game demonstrates a mind for game design and probability. In the standard card game of Duel Monsters, Duke's signature strategy also involves luck-based dice cards. His favorite and most powerful monster is Orgoth the Relentless, a fears warrior that appears in both his Dungeon Dice Monsters and Duel Monsters strategies. His other notable abilities are his incredible skill with physical dice manipulation, allowing him to perform impressive tricks and summon the exact crests needed to win in his own game.