Yūka Sasaki hails from rural Japan. Her journey into Circlet Bout began unexpectedly when she was mistaken for a challenger and thrust into a match against Japan's top-ranked player, Chikage Fujimura. Despite her complete lack of experience, Yūka displayed a natural aptitude, holding her own against the champion and forging mutual respect. This impromptu duel ignited her determination to compete in Circlet Bout.

Motivated by this resolve, she transferred to Tokyo's Saint Union Academy two years later to join its Circlet Bout program. Upon arrival, she discovered the school club had been deactivated. Committed to its revival, Yūka collaborated with new acquaintances—athletically gifted Miyuki Kasahara and tech-savvy Ayumu Aizawa—to reestablish the team, facing opposition from student council president Reina Kuroda. Her persistence proved instrumental when external challenges forced provisional approval for the club's reinstatement.

In Circlet Bout matches, Yūka wields a massive sword. Early in her training, she exhibited rapid skill acquisition, quickly mastering fundamentals. A key aspect of her combat ability involves exploiting the "Overheat Phenomenon," a tactical maneuver enabling simultaneous offense and defense—a capability shared with Chikage. This skill refined progressively through sustained training and competition.

Her personal background includes a family bakery business, which she consciously chose not to inherit, prioritizing her Circlet Bout ambitions. Known for discipline, she maintains rigorous training regimens and adheres strictly to protocols. She frequently corrects teammates' errors during practice and demonstrates strong convictions about fairness, openly opposing unethical match tactics. This commitment manifests in her self-proclaimed role as a "hero of justice."

Character development shows Yūka evolving from an inexperienced newcomer to a determined competitor facing progressively stronger opponents. Through tournament participation, she cultivated teamwork within her club while navigating interpersonal challenges, including complex rivalries with elite players like Chikage. Her leadership emerged organically as she motivated teammates through setbacks, balancing competitive drive with protective instincts toward fellow members.

Distinctive personal habits include frequently incorporating umeboshi (pickled plums) into her meals. Physically, she possesses long purple hair extending past her hips and blue eyes. She receives guidance from an AI bracelet named Siegfried, initially provided to enhance combat skills but occasionally offering moral support during challenges.