Hayashi Niko, who operates under the codename Sexy Voice, is a fourteen-year-old second-year middle school student with a uniquely developed set of skills. Her most prominent ability is an extraordinary control over her own voice, allowing her to mimic any voice she encounters, a talent that has earned her the title of master of seven distinct vocal personas. Complementing this is her heightened sense of hearing, which enables her to psychoanalyze people based solely on the tone, inflection, and quality of their voices, often deducing details about their personality, appearance, or emotional state.

Niko's background is unconventional, as she works a part-time job for a phone dating service, a role that involves using her vocal talents to keep men talking on the line, effectively running up their phone bills. This experience serves as a practical training ground for her deductive abilities, as she learns to read her clients through their speech patterns. Her professional life takes a turn when an older man, a former yakuza boss, recognizes her unique gift and begins hiring her as a troubleshooter or detective to solve various cases and mysteries. Her stated aspirations for the future include becoming either a spy or a fortune teller, professions that would both put her skills in understanding people to use.

In terms of personality, Niko is portrayed as quick-witted and precocious, possessing a sharp intelligence that allows her to navigate morally ambiguous situations. She demonstrates a genuine passion for solving mysteries and understanding how people think, finding a sense of thrill in the work. Despite her worldliness and her ability to manipulate others, she remains a child in many respects, and the stories often explore the tension between her sharp deductive skills and her incomplete understanding of the darker, adult world she is beginning to inhabit. Her nature is not purely altruistic; while she often ends up helping people, she is primarily driven by the intellectual excitement of the chase and the puzzle. Even with her talents, her deductions are not always immediately correct, but she persistently chases leads and follows hunches until she arrives at the truth.

Her key relationship is with Iichiro Sudo, a harmless and somewhat perverted adult mecha otaku whom she nicknames Robo. Sudo was originally one of her unwitting clients from the phone dating service. Niko easily manipulates him into becoming her partner and sidekick, and he often provides physical presence or comic relief, even though he is frequently confused about the details of the case at hand. Niko comes from a family that includes her older sister, Kazumi, a university student, her father Takeo, and her mother Yukie. Her life at home and her relationship with her sister provide a grounded counterpoint to her clandestine activities. The old yakuza boss who provides her with missions is another significant figure, serving as her mysterious benefactor.

Throughout the story, Niko demonstrates clear development as she uses her investigative work to learn important lessons about herself and human nature. She begins to find a sense of identity and purpose, navigating the fine line between her youthful innocence and the morally grey world of her cases without becoming fully cynical. Her notable abilities remain the core of her character: unmatched vocal mimicry, psychoanalytic skills based on auditory cues, and a sharp, deductive mind that she applies to solving mysteries.