Hiyori Kirishima, the only daughter of Zen Kirishima and his late wife Sakura Kirishima, is approximately ten to twelve years old and attends primary school. She has brown hair typically tied up with a pink scrunchie and brown eyes, bearing a noted resemblance to her deceased mother. Sakura passed away when Hiyori was an infant, leading to her being raised solely by her father.

She displays bright cheerfulness tempered by a maturity beyond her years, a trait developed from assuming significant household responsibilities like cooking, cleaning, and grocery shopping due to her father's inability to manage domestic tasks. This fostered early practical life skills and independence in adult environments. Her maturity also manifests as directness in expressing observations, a trait shared with her father. Following Takafumi Yokozawa's integration into their household, she gradually relinquished these duties and embraced more age-appropriate behaviors, focusing on school and peers.

Hiyori shares a close bond with her father, Zen, perceptively noting his emotional states and frequently scolding him for domestic mishaps. A shift occurred when she transitioned from calling him "papa" to "father" after facing teasing from friends, causing Zen distress. Her relationship with Yokozawa is notably positive from their first meeting; she affectionately calls him "Yokozawa-oniichan" or "onii-chan," reflecting a sibling-like dynamic. Their connection strengthens through shared interests in cooking and animals, regular emails, and her care for his cat, Sorata, when he could no longer provide adequate attention. She demonstrates deep investment, exemplified by her anger upon missing his birthday and organizing a belated surprise party. Yokozawa reciprocates protectively in a parental role, while Zen humorously labels Yokozawa the "mama" figure for undertaking maternal tasks like styling her hair and managing her clothing.

Hiyori knows little about her mother and expresses recurring curiosity. She has explicitly rejected the idea of Zen remarrying, stating she only desires a mother if her father genuinely wishes to remarry, emphasizing contentment with their existing family.

Within the "Valentine-hen" narrative, Hiyori facilitates interactions between Yokozawa and Zen. She urgently emails Yokozawa requesting help making homemade chocolates for Zen. After completing the task and going to bed, Yokozawa presents his own handmade chocolates to Zen, who then humorously pressures him for a verbal love confession, underscoring Hiyori's role in fostering their connection.