Kirie Kojima is a central character known for being the childhood best friend and next-door neighbor of the protagonist, Yukinari Sasaki. She is a high school student with brown hair and brown eyes. Physically, Kirie is notably well-developed for her age, a trait that is frequently remarked upon and which makes her a target for the perverted schemes of other characters; she is measured to have a G-cup bust size.

Her personality is defined by a volatile and aggressive temperament. Kirie is extremely quick to resort to violence, often operating on a principle of punching first and asking questions later, particularly when she believes Yukinari is acting in a lecherous manner. Her attacks are frequently executed with professional wrestling moves, such as a clothesline or an F5, and are often punctuated by her shouting a phrase that condemns the target’s actions. Despite this fierce exterior, Kirie secretly harbors deep romantic feelings for Yukinari, though she is completely unable to confess them openly. Her abusive behavior towards him ironically becomes a primary obstacle to developing a closer romantic relationship. She also displays a notable duality in her attitude towards perversion, as she sometimes shows little concern for nudity or being harassed when alone, but reacts with extreme violence if she believes Yukinari might perceive her as being immodest.

In the story’s early events, it is Kirie’s violent kick that sends Yukinari into a bathtub, which acts as a portal to the world of Seiren and serves as the catalyst for the entire plot. As Yukinari’s oldest friend, she often feels a sense of rivalry and jealousy towards the alien girl Miharu, who moves into Yukinari's home and for whom he develops strong feelings. Kirie also has a contentious relationship with the serial pervert Kazuharu Fukuyama, whom she constantly attacks to thwart his schemes, though she shows a rare, implied soft spot for him on the few occasions he acts heroically. Conversely, she demonstrates a protective, caring side towards the girl Kosame, who is infatuated with her. Even though Kosame’s seduction attempts irritate her, Kirie generally holds her in high regard and is willing to defend her.

Over the course of the narrative, Kirie undergoes noticeable character development. She begins as an exceptionally brutal and short-tempered individual but gradually becomes less violent and starts to show her compassionate side more openly as the series progresses. This growth allows her to become more honest with her own feelings and less prone to her initial, explosive reactions. In terms of abilities, beyond her formidable martial arts prowess and physical strength, Kirie has no supernatural powers, relying instead on her athleticism and combat skills to handle conflicts.