Rei Kugami, a central figure in the Flash storyline, protects the mysterious ARC pendant containing the Great Spirit of Duel Masters—a duty inherited through her family to shield it from the Nest organization. Initially distrustful and solitary in this mission, her path shifts when Teru Yumemi and his sister Cocoro intervene during a clash with Nest agents. This encounter leads Rei to bring them to her grandfather Kenshiro Kugami's dojo, where she both studies and assists in teaching Duel Masters.

A ninth-grade student possessing first-class dueling proficiency, Rei initially wields a Light Civilization deck, deploying Supernova Venus la Saint Mother as her trump card to defeat Teru in an online "Monster Battle Duel." Following a training duel loss against Cocoro Yumemi, she relinquishes Supernova Venus la Saint Mother and adopts a Water Civilization deck, making Supernova Neptune Strom her new primary card—marking a strategic evolution in her dueling approach.

Beyond personal competition, Rei mentors others at the dojo while steadfastly safeguarding the ARC pendant, bringing her into recurring conflict with the power-hungry Nest organization. A notable duel against Swan, another Water Civilization specialist, showcases her adaptability with the new deck. Throughout these challenges, Rei gradually learns to rely on allies, particularly Teru and Cocoro, shifting from isolation to collaboration against Nest.

Her dueling unfolds within a technologically advanced era where digital deck holders and virtual reality headsets replace traditional cards. These devices read card data to create immersive battles and also function as communication tools, representing a significant evolution from earlier Duel Masters eras. Within this setting, Rei's role culminates in the collective effort to thwart Nest's ambitions. After Teru becomes the Legendary Duelist and secures all Guardian Cards, Rei's ongoing mission to protect the pendant concludes alongside the organization's defeat.