Suzu Naito, a seventeen-year-old high school student in Kochi Prefecture, lives with her father. Her early childhood centered around a close bond with her mother, who fostered her passion for singing and composing music. This ended abruptly when Suzu, around age six, witnessed her mother perish saving a child during a flood. Traumatized and resenting her mother's choice to prioritize a stranger, Suzu suffered severe psychological distress. She lost her ability to sing, experiencing physical nausea upon attempting it, and emotionally distanced herself from her father and peers. This withdrawal persisted through adolescence, leaving her largely isolated except for a small circle of friends.

Suzu exhibits a reserved, introverted personality marked by persistent anxiety rooted in her mother's death. Her physical traits include freckled cheeks, russet brown eyes, and dark brown hair usually in a bob cut. She typically wears her high school uniform: a white shirt, red tie, and blue plaid skirt, sometimes adding a gray pullover and scarf in cold weather; casual wear consists of jeans and a white T-shirt. This contrasts starkly with her virtual avatar, Belle, generated by the "U" online world's biometric analysis. Belle possesses distinct features: knee-length rose-pink hair, blue eyes, linear coral-pink freckles, and an ethereal beauty. Her attire varies, including elaborate costumes like a red flower-themed dress during performances.

Suzu discovers "U" through her tech-savvy best friend, Hiroka "Hiro" Betsuyaku. Creating her avatar—initially named "Bell," translating "Suzu" (meaning "bell")—Suzu unexpectedly regains her singing ability within the virtual space. Her debut performance of "Gales of Song," remixed by Hiro as "Swarms of Song," rockets her to global fame in "U." Users rename her "Belle" (French for "beautiful"), cementing her status as a virtual diva. This persona unlocks confidence and artistic prowess absent in reality, though she directs all performance revenue to charity, reflecting enduring humility.

Belle's path crosses the formidable, reclusive "Dragon," persecuted by vigilante group "Justices" led by Justin. Intrigued by rumors of the Dragon protecting children, Suzu investigates his identity. Their initial "U" encounters are contentious, but Belle gradually discerns his vulnerability, aided by his companion, the "Angel." Concurrently, Suzu navigates real-world challenges: she harbors unrequited feelings for childhood friend Shinobu Hisatake, misinterpreting his protective pledge after her mother's death as romantic interest. She also mediates between classmates Ruka Watanabe and Shinjiro "Kamishin" Chikami after Ruka confides her feelings, dispelling Suzu's mistaken belief that Ruka favored Shinobu.

After Justin destroys the Dragon's castle and captures Belle, threatening to expose her real identity unless she cooperates, the Dragon intervenes, enabling her escape before vanishing. Suzu and Hiro intensify efforts to uncover his identity, discovering he is Kei, a junior high student enduring physical abuse with his younger brother, Tomo (the "Angel"). Contacting distrustful Kei proves difficult; he dismisses Suzu's claim of being Belle. Persuaded by Shinobu and her choir mentors—her mother's former associates—Suzu takes pivotal action: during a global "U" broadcast, she reveals her true identity as Belle and sings publicly as herself. This vulnerability convinces Kei to trust her, but communication is severed when his abusive father disconnects their internet.

Using environmental clues from Kei's livestream, Kamishin locates him in Kawasaki, Kanagawa. With authorities unable to intervene immediately under abuse protocols, Suzu travels alone to confront Kei's father. She physically shields Kei and Tomo during an altercation, facilitating their protection. This act catalyzes Suzu's emotional resolution: she comprehends her mother's selfless sacrifice, reconciling her long-held resentment. Returning home, she warmly reunites with her father at the train station. Shinobu acknowledges her newfound independence, releasing himself from his self-imposed guardian role. Suzu's healing culminates in regained singing ability and renewed connections with friends and family.

The duality between Suzu and Belle embodies a central theme of suppressed identity and liberation. Her journey illustrates how virtual existence enabled the expression of an obscured aspect of her self, ultimately facilitating real-world courage and self-acceptance. The name "Belle" and narrative framework draw intentional parallels to the Beauty and the Beast fairy tale, emphasizing themes of perception, inner beauty, and empathy across dual realities.