Nonohara Marin is a teenage female character distinguished by green eyes and waist-length brown hair. Hailing from an all-girls school, her life takes a pivotal turn when Dr. Narutaki selects her as the prime candidate for his plan to preserve Japanese masculinity. Deceived by the doctor, she dons a specialized costume engineered to adapt to male desires—a garment that binds her until she successfully collects 100 semen specimens.

Marin's personality blends notable naivety with persistent determination. Her lack of common sense prompts classmates to question her mental stability, yet she possesses a large heart and unwavering resolve. Despite frequently misunderstanding situations and her inexperience in romantic matters, she commits fully to her assigned task, refusing to yield to confusion or obstacles. The costume transforms her into a magical girl archetype operating under the motto "To love and serve," though her grasp of this role remains inconsistent.

A key dynamic involves her competition with Southpole One, a sexaroid created by the doctor's assistant. This rival interferes by depleting potential semen sources before Marin can collect them, forcing her into a race against the sexaroid's actions. Throughout this struggle, she receives instructional guidance from the doctor and his assistant on proper techniques for her mission, despite her initial lack of knowledge in such matters.