Rio Futaba is a second-year student at Minegahara High School and the sole member of its Science Club. Her extensive knowledge spans chemistry and quantum physics, and she often uses laboratory equipment to brew coffee during club activities. She possesses hazel eyes, long ash-blonde hair typically worn down, and wears glasses. During an instance of Adolescence Syndrome, a split version manifested with her hair in a ponytail and wearing contacts. She maintains a slender yet voluptuous figure with a large bust, a physical trait that caused significant self-consciousness due to unwanted attention during her early development.

Her personality blends bluntness, sarcasm, and scientific skepticism with underlying kindness and loyalty. Initially dismissive of Adolescence Syndrome as unscientific, her perspective shifted when she personally experienced the phenomenon. Intense self-loathing related to her body image during a summer break triggered a splitting into two distinct entities: one embodying her typical reserved self, and another seeking external validation through suggestive online posts. This duality resolved only after both versions acknowledged their shared identity and accepted their physical form, facilitated by Sakuta Azusagawa and Yuuma Kunimi. Post-reintegration, she retains her glasses but alternates between ponytail and loose hairstyles.

She grew up in an affluent household with parents frequently abroad, leaving her to manage the family home independently. An early formative experience involved Yuuma Kunimi offering her a chocolate cornet when she couldn't reach a crowded food truck, sparking her romantic feelings for him. Despite his subsequent relationship with Saki Kamisato, Yuuma continued running past the Science Club window during basketball practice for her benefit, and she reciprocated by watching his games. She eventually confessed to him but halted his response to avoid emotional distress.

Her relationships are pivotal. She maintains a close, intellectually grounded friendship with Sakuta, providing scientific insights for his investigations into Adolescence Syndrome cases. She tutors Sakuta's sister Kaede in mathematics, earning the privilege of being addressed by her first name. After high school, she enrolls in a national university as a science major and works part-time as a lecturer at the same cram school as Sakuta. Her role evolves into a consistent advisor for new Adolescence Syndrome cases, including Sakuta's multidimensional displacement in "Knapsack Kid," where she helps analyze his parallel reality experience with her trademark rational approach.

In the "Knapsack Kid" storyline, her scientific expertise remains central when Sakuta consults her about a scar potentially linked to Adolescence Syndrome. She advises him to seek insights from Shoko Makinohara regarding alternate realities. Later, when Sakuta becomes trapped in an idealized parallel world, her counterpart in that timeline confirms his suspicions about the universe's divergence through quantum mechanics discussions. Her calm demeanor persists even when confronted with multiverse theories, underscoring her stability as a confidante.

Her development across media shows continuity in her analytical mindset and supportive nature, transitioning seamlessly from a high school scientist to a university student and mentor while retaining her foundational traits of loyalty and intellectual rigor.