Hayase Nagatoro is the main female protagonist of Don't Toy with Me, Miss Nagatoro. She is a first-year high school student at Kazehaya High School, later advancing to the second year. Nagatoro has a slim, athletic build and a noticeable tan from her involvement in the swimming club, which leaves pale swimsuit tan lines. She is energetic, playful, and highly extroverted, often displaying a mischievous and teasing demeanor. Her personality is frequently described as a sadodere—mixing sadistic pleasure in provoking others with underlying romantic affection. She is possessive of her senpai and does not tolerate others bullying him, though she herself teases him relentlessly. Nagatoro is also athletic and skilled in martial arts, particularly judo, and has a competitive nature. She enjoys physical activities and is not afraid to assert herself.

Her primary motivation is to interact with Naoto Hachiouji, whom she calls Senpai. She discovers his hobby of drawing manga and begins to seek him out daily in the art club room. Her teasing is a way to gain his attention and elicit reactions, gradually revealing that she harbors genuine romantic feelings for him. Over the course of the series, her actions evolve from harsh bullying to more caring and protective behavior, though she maintains her playful edge. She often becomes flustered when her own feelings are addressed or when others try to get close to Senpai.

Nagatoro's role in the story is to serve as the catalyst for Senpai's personal growth. Her persistent teasing pushes him out of his shell, encouraging him to become more assertive and confident. She also acts as a source of emotional support, defending him from others and showing concern when she feels she has gone too far.

Key relationships include her close-knit group of friends: Maki Gamou (Gamo-chan), a tall, perceptive girl who often orchestrates situations to bring Nagatoro and Senpai together; Yoshi, a simple-minded and energetic friend who mimics Gamo; and Sakura, a sweet but flirtatious girl. Nagatoro is also close to her older brother Taiga and older sister Misaki, both of whom tease her about her feelings for Senpai. Her relationship with Senpai is the central focus of the series, developing from adversarial teasing to mutual affection, culminating in a confirmed romantic relationship by the series' conclusion.

Nagatoro's notable abilities include her exceptional athleticism, especially in judo, which she takes seriously and competes in. She also possesses sharp social intuition, reading Senpai's emotions effectively and using them to guide her teasing. One of her most prominent traits is her ability to provoke strong reactions; her psychological teasing is a deliberate tool for connection rather than malice. Throughout the story, she demonstrates growth in emotional vulnerability, learning to express her feelings more openly and becoming a supportive partner to Senpai as he pursues his artistic ambitions.