Natsumi Kurobe is an athletic woman and Futaba Igarashi's closest friend since junior high, when Futaba transferred to their Tokyo school. She possesses a striking appearance with short black spiky hair featuring disconnected side bangs, amber cat-like eyes, and a curvy, well-defined physique. Her style favors sporty and casual wear—dark leggings and tops during physical activities, or jeans, t-shirts, and sneakers in leisure time, occasionally accented by yellow and green nail polish.

Her perceptive nature blends protectiveness with mischief. She demonstrates fierce loyalty, particularly toward Futaba, often detecting Futaba's unspoken feelings for colleague Harumi Takeda before Futaba acknowledges them. Natsumi actively supports Futaba emotionally, taking her to restaurants or bars to uplift her spirits and creating opportunities for Futaba to spend time alone with Takeda. She playfully teases Futaba by photographing her in embarrassing moments and encouraging her romantic pursuits. However, she harbors deep disdain for superficiality, especially from men who approach women based solely on appearance—a trait stemming from a past relationship where she abruptly ended things and slapped an ex-boyfriend after learning he dismissed a previous girlfriend for having smaller breasts. This intolerance leads her to ignore or physically rebuff insincere advances, such as kicking a persistent admirer.

She enjoys gaming, frequenting arcades and inviting Futaba to play fighting games together, and consumes alcohol socially, occasionally drinking vodka. Her social circle expands through the Sakurai siblings; after helping Touko Sakurai's younger brother Yuuto, she befriends Touko and visits their home. Natsumi assists Yuuto with sports, shares outings and gaming sessions, and engages in playful teasing—like dressing him as Little Red Riding Hood while she wore a wolf costume during a cosplay event. Though Yuuto develops romantic feelings for her, she views him platonically as a younger brother, maintaining a dynamic tinged with light sadism.

Within the narrative, Natsumi catalyzes Futaba's emotional growth. During a pivotal conversation, she guides Futaba to recognize her romantic feelings for Takeda by posing pointed questions about Futaba's reactions to Takeda's comments on companionship, then urging independent reflection that sparks Futaba's love epiphany. She also bridges her longstanding friendship with Futaba to workplace relationships, fostering social connections among colleagues.