Sugoroku Mutou (武藤 双六, Mutō Sugoroku?) is the grandfather of Yugi Mutou, who gave him his famous Millennium Puzzle as a present, which he had recovered from the tomb of the Pharaoh Atem in his youth. He owns the Kame Game shop, where Yugi and his friends get several of the games they play, and he taught Katsuya Jonouchi how to play the Duel Monasters card game. His fondness for games is even evident in his name: "Sugoroku" is a Japanese game similar to Backgammon. Sugoroku is the reincarnation of Ancient Egyptian vizier, Siamun Muran, right hand man to the Pharaoh Atem.

In the original manga, Sugoroku’s ownership of the rare Blue Eyes White Dragon card led him to be targeted by Seto Kaiba, who stole the card from him and duelled Yugi with it. Yugi reclaimed the card, but Kaiba, seeking revenge, forced Sugoroku to the dueling stage of his KaibaLand amusement park and defeated him, tearing up his precious Blue Eyes White Dragon card. While simply holding the character hostage in the Toei anime, in the original manga, Kaiba took his revenge even further by subjecting Sugoroku to virtual reality torture in order to force Yugi to enter “Death-T”, a series of games and amusement park rides intended to kill him, in exchange for Sugoroku's life. In the end, Yugi defeated Kaiba and Sugoroku was saved. The Duel Monsters anime reimagined and combined these two separate events into one, as Kaiba did not steal the Blue Eyes White Dragon, but kidnapped Sugoroku as his first move and defeated him in a duel, refraining from any further torture.

Things only got worse for Sugoroku when Maximillion Pegasus removed his soul from his body as a means of forcing Yugi into competing in his tournament. The original manga, he trapped Sugoroku’s soul in a video camera, while in the Duel Monsters anime, he imprisoned in within a Duel Monsters card. Sugoroku was freed when Yugi defeated Pegasus at the conclusion of the tournament, and continued to dispense support and advice for Yugi in his subsequent battles and duels.

The Duel Monsters anime gave Solomon Muto the chance to get back into the action during the KaibaCorp Grand Prix, which he participated in under an alias. In the Japanese version he uses "Mask of Rock," while in the English dub of the series, he uses the alias "Apdnarg Otum", "Grandpa Muto" backwards. He duels Joey using an ancient-themed deck with a lot of rock monsters; Joey defeats him.

Sugoroku reappears in episode 75 of Yu-Gi-Oh! GX, where he is kidnapped by Thunder (Ikazuchimaru) and Frost (Kourimaru).