Horie Yukiko attends Reverse Cross High School in post-apocalyptic Tokyo. Frequently described by peers as unattractive and chubby, she nonetheless demonstrates consistent kindness and boldness in her actions. Witnessing the protagonist's combat abilities against the city's monsters sparks her romantic feelings for him. Her background includes being childhood friends with Hiroshi Haoka, a feared delinquent harboring resentment towards anyone pursuing her. Yukiko maintains friendships within the school, including with Aoki, whose face is obscured by unexplained bandages. A significant turning point occurs when antagonist Chidokuro kidnaps Yukiko. During captivity, she undergoes forced transformation into a Tactical Fiend, a monstrous entity reflecting psychological distortion. Designated "Tsumiko" in this state, her body grotesquely mutates: taking on a caterpillar-like form that endlessly repeats the word "sticky" and sprouting adhesive tentacles capable of stripping flesh on contact. This form also wields projectile forehead beams. Further mutation reveals a secondary, wasp-like combat mode featuring a prominent stinger, later clarified as a parasitic appendage rather than inherent biology. The transformation proves reversible; physical contact from the protagonist restores her human form. Post-reversion, she resumes normal activities at school, with no explicit mention of lasting physiological consequences. Her narrative primarily focuses on the trauma of her transformation and subsequent recovery within the school environment.