Kasumi Oshino is a high school student working as a waitress in her school’s converted maid cafe. She persistently addresses protagonist Kazuya as "Darling" while vying to become his girlfriend, sparking fierce rivalry with childhood friend Natsuki. Despite their long history, Kasumi and Natsuki clash relentlessly over Kazuya’s attention, their arguments escalating into physical fights fueled by their opposing family legacies—Kasumi’s ninja lineage versus Natsuki’s samurai roots—resulting in chaotic destruction.

Physically, Kasumi stands 156 cm tall with a slim build, pale skin, and small breasts (B cup, measurements 78-53-83). Her shoulder-length grey hair features blunt bangs and a side part, paired with amber tareme-shaped eyes. She frequently wears school uniforms, maid dresses with headdresses, thigh-high stockings, miniskirts, and occasional ninja suits, accessorized with forehead necklaces, ribbons, or sashes. As a kunoichi trainee, she employs kunai, shuriken, throwing knives, and wire in her practice, specializing in an "exploding leaves" jutsu.

Personality-wise, Kasumi exhibits a cheerful, energetic, and deredere demeanor, often appearing kind yet displaying assertive, selfish, or rude tendencies. Her clumsiness surfaces during ninja training. She belongs to Natsuki’s social circle and enjoys hobbies like reading romance novels and browsing the internet.