Isami Narashino, a high school student and classmate of the protagonist, operates under the alias Yumi. She possesses distinctive blond shoulder-length hair with spiky bangs and red Tareme-style eyes. Her physique is slim and pale, standing 148 cm tall with measurements of 85-56-78 cm and an E-cup bust size.

Her personality presents a stark conflict: timidity, cowardice, and profound distrust clash with violent tendencies and an "Atai" (bold/daring) nature. This duality manifests in frequent stalking behavior and alcohol consumption.

Her attire consistently reflects her interests, blending military elements like uniforms, berets, helmets, and goggles with school-related clothing such as miniskirts, knee-high socks, ribbon ties, school swimsuits, uniforms, shorts, sport bloomers, sports uniforms, and tank tops. She is rarely seen without military items, often carrying grenades, handguns, or machine guns.

She is a member of Maaya's friend group and participates in the Military Study Group club. Biscuits, hardtacks, and crackers rank among her favorite foods. Notable weak points include a pronounced fear of insects, animals, and Russians. The phrase "I-I'm surprised, that bastard... is there!!" is frequently associated with her.

The character engages in or is subjected to specific activities including bondage, interrogation, and torture scenarios.