Hayase Nagatoro is a first-year student at Kazehaya High School who serves as the central driving force of the series. She initially appears as an ordinary high school girl, but her defining trait is her love of teasing her upperclassman, Naoto Hachiouji, whom she calls Senpai. This teasing is playful and often mischievous, but it is underpinned by genuine interest and affection. When interacting with others outside of Senpai, she generally behaves in a kind and normal manner, suggesting that her provocative behavior is reserved specifically for him.

Nagatoro has a bold and energetic personality. She is confident, assertive, and enjoys pushing boundaries, particularly when it comes to getting a reaction out of Senpai. Her actions are frequently described as those of a tsundere, as she often denies her growing feelings for him while simultaneously displaying possessive tendencies. She becomes visibly uncomfortable or jealous when other girls, including her own friends, get too close to Senpai. Despite her outwardly tough and teasing exterior, she has a caring and supportive side that emerges more frequently as the story progresses. Her teasing often serves as an unconventional form of encouragement, helping Senpai to step out of his shell and build confidence.

Nagatoro's primary motivation is her desire for connection with Senpai. While she derives entertainment from toying with him, her actions are driven by a wish to understand him better, to see him grow, and to become closer to him. She struggles to express her feelings in a straightforward manner, so her teasing becomes her primary method of interaction and communication.

In the story, Nagatoro acts as the catalyst for Senpai's personal development. Her persistent presence in the art club room and her constant prodding force him to confront his social anxieties and his insecurities about his art. Her role shifts from being a mere tormentor to a source of motivation, pushing him to take on new challenges and become more self-assured.

Key relationships include her dynamic with Senpai, which forms the emotional core of the series. It evolves from one-sided torment into a mutual exchange where both characters influence each other's growth. She is also close with her group of friends: Gamo-chan, Yoshi, and Sakura. These friends are often aware of Nagatoro's true feelings and act as supporters and instigators, sometimes teasing her in return or creating situations that force her and Senpai to interact more closely.

Over the course of the second season, Nagatoro exhibits notable development. She becomes more openly affectionate and vulnerable, showing moments of genuine care and concern for Senpai's well-being. She also displays a softer, more insecure side, particularly when confronted with her own past failures or the possibility of losing Senpai's attention to someone else. The season deepens her character by exploring her history with judo, revealing that she quit the sport after facing a rival she could not defeat, which parallels her own struggles with perseverance and vulnerability.

Regarding notable abilities, Nagatoro is physically active and skilled in martial arts, particularly judo. She has a natural athleticism and a competitive streak, which she occasionally showcases in sports events or sparring sessions. However, her most significant ability is her sharp intuition; she is adept at reading Senpai's emotions and understanding his fears and desires, which allows her to tailor her teasing in ways that ultimately help him grow rather than simply hurt him.