Ouka Shiunji is the third daughter of the Shiunji family, a sixteen-year-old second-year high school student who lives with her six siblings in a mansion in the Setagaya district of Tokyo. She is the same age as the family's eldest son, Arata Shiunji, and the two are raised as twins, forming the third oldest sibling pair in the household. Ouka has light pink hair worn in long twin ponytails that reach past her elbows, tied with blue ribbons, and bright blue eyes. She typically wears her school uniform with a pale pink cardigan, and her birthday falls on October 10.

In personality, Ouka is outgoing and highly sociable, easily making friends with people of any gender and earning widespread goodwill among her classmates. She is kind-hearted toward most people, though she tends to be sharp and feisty with Arata, and the two regularly butt heads. Despite their bickering, they share a close and meaningful bond; during childhood they had so many moments of perfectly synchronized thoughts and actions that their friends assumed they must share some kind of twin psychic connection. Ouka trusts Arata deeply, and her relationship with him is one of the most important elements of her character.

Ouka is a hardworking and determined person who places great value on achievement. She ranks second in her school academically, behind only her older sister Seiha, and her consistently high standing is credited largely to her effort and diligence rather than natural ease. She is equally accomplished in sports, serving as the ace of the school swim team while also excelling in track and field. This combination of academic and athletic success makes her a well-rounded and respected figure at school, and she is regarded as the frontrunner for the next student council president election.

Within the family, Ouka acts as an energetic and uplifting presence, striving to bring a bright and positive spirit to her relatives and friends. Her motivations center on self-improvement and on the people she cares about: she works hard to maintain her high standing, supports those around her, and treasures her bond with Arata as something special.

Her story arc is closely tied to the revelation that the seven Shiunji siblings are not actually related by blood, a secret disclosed by their father. As the children come to terms with the fact that their family bond is not what it always seemed, Ouka confronts her complicated feelings for Arata. She eventually confesses to him, acknowledging that others may struggle to accept it, but that she sees him as both her brother and the person she likes. That confession marks a significant turning point in her development, demonstrating her willingness to face difficult truths and act on her emotions despite the risk of rejection or disapproval.

In terms of abilities, Ouka is a high achiever across academics, athletics, and social leadership. She is the ace of the swimming team, a top performer in track and field, one of the top students in her school, and a natural leader whose popularity and interpersonal skills make her a leading candidate for student council president.