Hiyori has brown hair that she usually keeps up in a scrunchie along with matching colored eyes. Hiyori is said to resemble her late mother, Sakura, more than her father.
Hiyori is a bright, cheerful young girl who is described to be rather mature despite her young age, according to Yokozawa. This is likely due to her being left to tend to the household chores and meals for her and her father on a day-to-day basis for a majority of her life, including doing the household and grocery shopping. As a result of this, in the light novels, she is shown to be quite capable of navigating on her own in the adult world. Her mature nature is shown to even surprise some adults, given her young age. In the novels, similar to her father, Hiyori is an open-minded girl who can be somewhat direct with her observations of facts or opinions, something acknowledged by both Kirishima and Yokozawa.
However, following Yokozawa entering her and her dad's lives, she is gradually able to behave more like a child her age. Due to Yokozawa slowly taking over the household chores, Hiyori is finally able to let go of her prior responsibilities bit by bit. As this happens, she is finally able to focus on enjoying her life as a kid and becomes more social among her peers now that she has the freedom to do so without her former worries.
Zen Kirishima is Hiyori's father. They appear to share a very close father-daughter relationship, clearly due to him raising her as a single father since she was a baby. Hiyori can often sense if her father is feeling upset and can easily tell when something is bothering him. Zen's inability to cook or do proper housework caused Hiyori to take on household related chores such as cooking, cleaning, and grocery shopping. Since most of the housework is Hiyori's responsibility, Hiyori is usually seen scolding Zen for either making a mess or buying the wrong items from the store.
Hiyori recently began calling Zen "father" instead of "papa" due to her friends teasing her about it. Zen appears to be upset at the thought of Hiyori growing apart from him as she gets older and becoming her own woman.
When Hiyori and Yokozawa were first introduced to each other, she took an instant liking to him. This, in which, surprised Zen given how scary of a reputation Yokozawa has and how he usually intimidates people. They bond over cooking, their interest in animals and various other things. Hiyori and Yokozawa are seen e-mailing each other regularly, mainly in the manga and light novel. Yokozawa had even given Hiyori his cat, Sorata, due to being unable to regularly care for the animal himself. Hiyori seems to trust Yokozawa to a large extent, usually telling him about her daily activities through text messages throughout the day before being able to talk to him in person after school. It is shown that Hiyori also sometimes makes Yokozawa lunches with cute designs, which slightly embarrasses him at work.
Also, Hiyori is shown to be close enough to him that when she found out that Yokozawa's birthday had passed by, she became deeply angry with him for not telling her. This being on account of wanting to have thrown him a party for the special day and gave him the cold shoulder for a brief period, which actually got to Yokozawa. However, she soon gets over her anger by throwing him a surprise belated birthday party instead. Hiyori calls Yokozawa "onii-chan", translating to "big brother", because of their closeness.
Yokozawa has also grown close enough to Hiyori that he admits to viewing her like a daughter, even holding a protective sense over her like that of a parent. Likewise, Kirishima also comes to view Yokozawa as the "motherly" figure to his daughter, especially since Yokozawa often does Hiyori's hair before she goes to school and handles her clothes. Hence, Kirishima will teasingly call him "mama" due to his motherly nature towards Hiyori.
Hiyori, who seems to also share Yokozawa's love for animals, cares deeply about Sorata. She had always wanted a cat of her own, but didn't think she was allowed to have one. When Yokozawa realized he couldn't care for Sorata like he previously could, Hiyori immediately offered to care for the animal. Knowing Sorata was better left in Hiyori's care since he would be given more attention, Sorata was given to Hiyori. She frequently cares for him and often has him sleep in her bed beside her.
- Hiyori does not appear in the anime. However, she makes an appearance in the movie focused on Yokozawa Takafumi.
- While not appearing in any of the anime, except in the anime film, she often makes regular appearances in the manga and also regularly appears in the light novel that is focused on Yokozawa.
- Due to her mother dying when she was a newborn, she knows almost nothing about her. Hence, she often inquires about her whenever the situation allows for it and wishes to know more about her.
- She does not know about her father's true relationship with Yokozawa since both men prefer she not know given her young age. However, Kirishima fully intends on telling her when she is older and firmly believes that she will have no trouble accepting it since he is raising her to be open minded.
- Hiyori doesn't hold the desire for a new mother, possibly even against the idea of it. In the light novels, when a classmate suggested the idea of her father re-marrying, she became deeply upset. When her father and Yokozawa asked how she felt about gaining a mother, she responded that she didn't want a new mother as she already has the family she needs. She further said that she would only want a mother strictly if her father ever did.