Rio Nanase is the second oldest of five sisters. She possesses long blue hair usually arranged in twin tails, violet eyes, stands 158 cm tall, and has measurements of 90-59-88 cm with an E-cup bust. Her typical outfit is a school uniform featuring a cravat, pleated skirt, thigh-high stockings, and a ribbon hair tie; she occasionally wears glasses or hairpins.

Personality-wise, Rio embodies a modern tsundere archetype. She is quick-tempered, strict, and outwardly arrogant, often reacting with violence like elbowing or punching, especially toward her older brother when flustered. Despite this hostile exterior, she harbors deep affection for him and is genuinely concerned about his well-being. Academically focused, she takes responsibility for overseeing her younger sisters' studies. Rio is highly sensitive and easily embarrassed, particularly regarding sexual topics, which cause her to become flustered and red-faced, lagging behind her sisters in this regard. Her inability to honestly express feelings frequently leads to aggressive outbursts masking genuine care or affection.

Her activities include shopping and swimming. She exhibits possessiveness over her older brother's attention, feeling displeased when he interacts romantically with their other sisters. Her relationship with him is complicated by her tsundere behavior; she consistently appears angry in his presence but struggles to convey her true feelings positively, addressing him as "Aniki."

In the generational sequel, Rio becomes the mother of Ririna Nanase. Her established role remains that of a tsundere sibling balancing academic diligence with concealed emotional vulnerability.