Atsuko Yamaguchi attends Arashiyama High School as a female human student, marked by auburn hair and petrol-colored eyes. She does not wear glasses, lacks freckles, and abstains from smoking. Her height, weight, and blood type remain unspecified.

She shares a close friendship with classmate Takeshi Chimatsuri, though subtle indications point to deeper romantic feelings on her part. When the Toki twins transfer to their school and challenge Takeshi to reclaim their family dagger, they exploit his intense phobia of the naked female body. Atsuko intervenes by designing and participating in specialized resistance training for him. She physically demonstrates desensitization techniques, employing direct bodily contact during sessions.

Throughout the training, Atsuko observes Takeshi's reactions, noting how his extreme gratitude for minor assistance momentarily overcomes his phobia. This insight shapes her approach, driving her to commit fully and use her own body as a tool to build his resilience against the Toki twins.

Her occupation is explicitly that of a student. Family relationships, pets, special abilities, and involvement in media beyond the original OVA are not documented.