Nagumo is a gamer with interest in a variety of games, ranging from Monster Fighter to Duel Monsters. He engages in games to make money and often focuses on the potential resale value of desirable collectibles, including Monster Fighter figures and "Obelisk the Tormentor". Nagumo is a bully, relying on his physical strength to win games when he is in danger of losing, and using his hobbies to lure in potential victims to extort with his strength.
Nagumo was unhappy with the card he has won through the ante rule after one of his Duels. He demanded that his opponent give also him his second rarest card.
However, Mokuba Kaiba appeared and calls a foul for changing the ante. Koji insulted Mokuba, calling him "Junior". However Seto Kaiba appeared and told Nagumo off, saying that speaking rudely to Mokuba dishonors Kaiba and the Battle City tournament. Kaiba challenged him to a Duel, anteing a briefcase of rare cards and "Obelisk the Tormentor". He allowed Koji to use the cards from the briefcase in the Duel, but said he would take them back after he won. Koji, overconfident at his newly empowered Deck, suffered a devastating defeat as Kaiba unleashed "Obelisk", and took his Locator Cards. Koji was left traumatized by Obelisk's overwhelming power, and wound up screaming in terror as he relived the image of the God crushing him.
During the finals, Nagumo observed the fight between Yugi and Kaiba from television screen in Domino City, along with other Duelists that had been eliminated from the tournament.