Rio Futaba, a second-year Minegahara High student, diligently maintains the Science Club as its sole member, even during summer breaks. She possesses hazel eyes, long ash-blonde hair typically worn loose, and wears glasses. Early physical development left her with a voluptuous figure, causing profound self-consciousness that fuels her character journey. Her parents' frequent work abroad forces her to manage household responsibilities alone. In her first year, classmate Yuuma Kunimi sparked enduring romantic feelings when he gave her a chocolate cornet after she struggled to purchase food in a crowd. Though Kunimi later dated Saki Kamisato, he continues basketball practice near the Science Club window for Rio's benefit, and she regularly watches him play. Adolescence Syndrome manifests during summer break when intense body insecurity and isolation—exacerbated by Sakuta and Kunimi's relationships—trigger quantum entanglement. This splits Rio into two entities: one retains her original appearance (glasses, lab coat, reserved demeanor) while the other embodies desired confidence (contacts, ponytail, assertive personality). The confident version posts suggestive photos on Twitter as "@choco_cornet," referencing Kunimi's gesture, seeking validation. Both versions hide the syndrome's origin from Sakuta but acknowledge each other's existence. The division escalates internal conflict. The original Rio becomes despondent watching her confident self pursue Kunimi, viewing it as personal failure and contemplating self-erasure. Meanwhile, the confident Rio arranges a fireworks viewing with Kunimi. The crisis peaks when the original Rio, after confessing despair to Sakuta, calls her confident self from a hospital payphone to request joining the event. This acceptance merges both entities, integrating confidence into her core identity and signifying self-worth beyond appearance. Post-merger, Rio keeps her glasses but alternates hairstyles, reflecting her unified self. She openly assists Sakuta with cases involving Shoko Makinohara, applying quantum physics expertise to supernatural phenomena. Academically, she advances to the National School of Sciences, later working as a cram school lecturer during university. Her relationships showcase complexity: with Sakuta Azusagawa, she shares blunt, sarcastic exchanges while aiding his investigations despite initial skepticism about Adolescence Syndrome. She tutors his sister Kaede, who uses her first name. Interactions with Mai Sakurajima begin during Rio's syndrome incident, evolving into mutual respect and collaboration; Rio retains Mai's contact for future cooperation.