Masumi is the representative for the Leo Institute of Dueling Fusion Summon course. She uses a "Gem-Knight" Deck. Masumi is a direct, straightforward and bold individual, showing little to no hesitation in saying what she believes. Due to her father's job and witnessing and appraising the value of jewels, she is seemingly capable of detecting talent and hesitation within people, as she did with Yuzu at the start of their Duel. Although Masumi shows a penchant for mockery, most of what she states is accurate thus, she does not act out of plain arrogance but on observation. Masumi is also a caring person, valuing her friends and her teachers, and after Professor Marco's disappearance, she became frantic in her attempts to find him. She cares for (and possibly has a crush) on Marco particularly, and doesn't tolerate insults towards him.

Thanks to the Leo Corporation, headed by Reiji Akaba, new advancements to Solid Vision, the holographic system that gives life to the Duel Monsters card game, give physical mass to monsters and environments, allowing players to interact directly with them. This has given birth to challenging Action Duels, in which duelists may traverse environments to locate Action Cards to help them in a pinch. The story follows Yuya Sakaki, a second year middle school student at Miami City's You Show Duel School, whose father disappeared three years ago when he was scheduled for an important duel. While facing off against the current Action Duel champion, Yuya awakens a new power from his pendant, known as Pendulum Summoning.

Masumi Koutsu