Rio is a stoic and blunt girl who possesses substantial knowledge in science, including chemistry and quantum physics. She has a sharp sense of humor and is even more sarcastic and straightforward than Sakuta. Before experiencing it herself, she was unwilling to believe the existence of Adolescence Syndrome, but she did acknowledge the abnormal phenomenon it induces and connects them with various scientific theories and principles. Despite her stoic nature, Rio also shows various emotions such as love, as holds affection towards Yuuma, as well as anxiousness. She's also genuinely kind and loves her friends.

Rio is implied to have experienced a form of neglect, as shown when Sakuta visits her house, she tells him her parents are usually abroad because of work. In addition, she has no siblings, which further proves that she chooses to be alone as this is her comfort zone that she unwillingly developed thanks to her lonely childhood. Although, this loneliness is what gave her the courage to approach Sakuta first when they were in the same class, as she saw him as an outcast just like her.

The second Rio Futaba behaves nearly identically to the original, but she has taken a hobby of posting risque photos on social media as a form of "self-mutilation" and to have an objective look on herself.