Rio Yamamoto becomes Akari Yamamoto's stepbrother when their parents marry just before the series starts. He concealed his secret romantic feelings for Akari to protect their new family's stability. During an attempt to confess, Akari intuited his intent and feigned breaking her phone to halt him upon unexpectedly seeing their parents together. This unspoken affection created underlying tension, especially as Akari's mother remained wary of Rio spending time alone with her daughter.

Rio appears calm and laid-back, often choosing his passion for photography over romance during high school. He connects meaningfully with Yuna Ichihara after stopping her from stepping into dog feces; she becomes infatuated due to his resemblance to a prince from her favorite manga. Rio confides in Yuna about his unresolved feelings for Akari, and she supports him emotionally. Through these interactions, Rio gradually develops genuine romantic feelings for Yuna.

Rio's character evolves as he confronts his emotions. During a cultural festival, resolved to confess to Yuna after learning his friend also intended to confess to her and hearing rumors about her planned confession to someone, he searches for her. Simultaneously, Yuna resolves to confess to Rio. Their frantic search culminates in Rio confessing first, expressing his need to declare his feelings before she confessed to another. Yuna clarifies she intended to confess to him all along, leading to their mutual declaration and relationship.

After this, Rio and Yuna maintain a committed relationship. By the manga's conclusion, they attend college together while continuing to date. Rio's journey centers on overcoming past emotional constraints, embracing vulnerability, and committing to a reciprocal romantic partnership with Yuna. His name, meaning "reason, logic" (理) and "center" (央), reflects aspects of his personality and narrative role.