Hana Sunomiya is a supporting character who appears in the second season of the anime, introduced as a new first-year student joining the art club during Senpai's third and final year of high school. She is the younger cousin of Sana Sunomiya, the former art club president, and shares a striking physical resemblance to her, though she has shorter dark purple-black hair in a bob cut, purple eyes, and slightly rounder cheeks that emphasize her younger age. Hana stands as a very tall and beautiful young woman with pale skin, and she is typically seen in her school uniform, which consists of a white button-up shirt with a red tie, a long navy blue skirt, and sneakers. Her connection to Senpai predates her arrival at Kazehaya High School, as she knew him from middle school where they both participated in the art club, and she holds a great deal of respect for him as her senior who taught her much about the arts.

On the surface, Hana often appears cold, stoic, and serious, rarely smiling and maintaining a composed expression. However, her true personality is far more dynamic. She is extremely polite and well-mannered, consistently bowing to older students and raising her hand before asking questions to ensure she has permission to speak. Despite being prone to fretting about sounding rude, she is remarkably honest and direct in her observations and questions. Hana is also surprisingly passionate, particularly when it comes to the subject of romance in general and the relationship between Senpai and Nagatoro specifically. She becomes an immediate and fervent supporter of their pairing, viewing Nagatoro as a goddess who transformed Senpai's art style from the more muted works of his middle school days into something far more colorful and vibrant. This passion often manifests in dramatic outbursts as she reacts to new developments in their relationship, displaying more enthusiasm and urgency than even Senpai himself.

Hana's primary motivation is to actively cultivate the romantic connection between Senpai and Nagatoro. She takes on a role akin to a self-appointed romance advisor or drill sergeant for Senpai, constantly monitoring his progress and giving him instructions on his next moves. She is the character who pushes him to ask Nagatoro on their first date to the aquarium, and she later advises him on specific actions like holding hands to test if his feelings are mutual. Because she acknowledges her own lack of personal romantic experience, Hana derives most of her advice from the tropes and scenarios she has absorbed from reading romance manga. While initially viewing Nagatoro as a goddess, she later adopts Senpai's own nickname for her, calling Nagatoro and her friends demonesses, though she never wavers in her support of their relationship.

In the story, Hana's role is multifaceted. As a kohai to all the other main characters, she is the youngest member of the group and introduces a fresh dynamic to the art club. She becomes an unexpected but effective facilitator for the central romance, sometimes taking physical action to help, such as tailing Nagatoro's friends Gamo-chan and Yoshi during the aquarium date to prevent them from interfering with Senpai's romantic pursuits. Her key relationships are defined by these dynamics: she has a deep respect for Senpai as her artistic mentor, an admiring view of Nagatoro as a positive influence, and a close bond with her cousin Sana, whom she resembles not only in looks but also in habits. One notable shared trait with her cousin is an apparent nonchalance about nudity, as she has shown a willingness to undress to pose as a model without much hesitation, though she may be slightly less experienced in this regard than Sana, occasionally blushing as she does so.

Throughout her time on the show, Hana shows subtle signs of development. While she begins simply as a very perceptive observer who instantly identifies the romantic tension between Senpai and Nagatoro, she evolves into a more active agent in the story. She transitions from giving general encouragement to providing concrete, step-by-step plans for Senpai to follow, demonstrating a growing investment in seeing their relationship succeed. Her development is less about personal change and more about her increasing effectiveness and determination as what could be called the story's internal romantic coach. Hana's notable abilities include being a fast learner and an extremely talented artist, as well as having extraordinary cooking skills that she once demonstrated by preparing a bento box for Senpai. She is also an avid reader of romance manga, which serves as her primary source book for relationship advice, and she possesses remarkable perceptiveness and deductive abilities, quickly piecing together situations from minimal information.