Hiyori Kirishima is the young daughter of Zen Kirishima and his late wife, Sakura Kirishima. First introduced at the age of ten, she is a primary school student with brown hair that she typically secures with a pink scrunchie and matching brown eyes. She is said to resemble her deceased mother more than she does her father. Her mother passed away when Hiyori was an infant, leaving her to be raised solely by Zen.

Hiyori possesses a bright and cheerful disposition, but she is also described as being notably mature for her age. This maturity is largely a product of her circumstances, as her father is incapable of cooking or managing household tasks. Consequently, Hiyori took on the responsibilities of cooking, cleaning, and even grocery shopping from a very young age. This has made her surprisingly capable of navigating adult environments independently, a trait that often surprises grown-ups. Like her father, she can be quite direct and open-minded in her observations.

The arrival of Takafumi Yokozawa in her life marks a significant turning point for Hiyori. As Yokozawa gradually takes over the domestic chores, Hiyori is slowly relieved of her burdens and given the freedom to behave more like a child her age. This allows her to focus on having fun and becoming more social with her peers, something she had little time for previously.

Hiyori shares an exceptionally close bond with her father, Zen. She is perceptive of his emotional state and can easily tell when something is bothering him. Because she has long been the one managing the household, she is often seen scolding Zen for making messes or purchasing the wrong items at the store. A subtle shift in their relationship occurs when she begins calling him father instead of papa, a change prompted by teasing from her friends, which causes Zen some private distress over the thought of her growing up.

Her relationship with Takafumi Yokozawa is a central part of her character. When they are first introduced, she takes an instant liking to him, which surprises Zen given Yokozawa's intimidating reputation. They quickly bond over shared interests like cooking and a love for animals. Hiyori trusts Yokozawa a great deal, regularly emailing him about her day. Their connection is so strong that when she discovers his birthday has passed without her knowledge, she becomes deeply angry and gives him the cold shoulder before eventually throwing him a surprise belated party. She affectionately calls him onii-chan, meaning big brother. Yokozawa comes to view Hiyori as a daughter, and in turn, Zen sees Yokozawa as a motherly figure in Hiyori's life, especially since he helps with tasks like styling her hair and choosing her clothes. Zen even teasingly calls Yokozawa mama because of this nurturing nature.

Hiyori also cares deeply for the cat Sorata, which originally belonged to Yokozawa. She had always wanted a cat but did not think she was allowed to have one. When Yokozawa could no longer care for Sorata properly, she immediately offered to take him in, and the cat now frequently sleeps in her bed.

Over the course of the story, Hiyori develops from a serious, self-reliant child forced into adulthood too soon into a girl who can finally enjoy her youth, thanks to the new family dynamic. She remains, however, a perceptive and caring individual who is the emotional heart of her household. A notable ability is her skill in cooking, which she uses to prepare meals for her father and occasionally make cute, designed lunches for Yokozawa to take to work. She is unaware of the true romantic nature of her father's relationship with Yokozawa, as both men have decided to wait until she is older to tell her.