Masumi Momono 「百野真澄, Momono Masumi」 appears only in a special chapter of Yu-Gi-Oh! R. He, along with his two lackeys, form the group known as "Store Breakers", duelists who go from store to store, taking control of them and stealing rare cards.
Like the Card Professors, Masumi Momono is named after the codename for a microchip: the 1.266 GHz Flip Chip Pin Grid Array (FCPGA2) Pentium III (P3-850) microprocessor.
He, along with his two lackeys show up at Suguroku's game store during a tournament, the prize of the tournament being a Duel with Yugi. While Yugi waited outside, he met some children asking him to halt the tournament, and they explained the situation with the "Store Breakers" and what they try to do. Yugi then busts in just as one of Momono's henchmen defeats Jonouchi, and though they feign innocence at first, Momono quickly rises to challenge Yugi. Masumi sets the condition that, should Yugi lose, Masumi gets the store's profits, in addition to the Egyptian God Cards. Yugi accepts, on the condition that, if he wins, Masumi loses all of his cards.
Masumi pulls out a "Light" Deck to counteract Yugi's "Dark" deck, but ultimately loses after a long and difficult Duel.
Masumi possesses numerous thematic Decks (his name translates loosely to "100 fields"), and uses them according to the opponent that he's facing. Since most of Yugi's strongest cards are related to the DARK monsters, Masumi chooses a LIGHT Deck which to Duel against him.