Naoko Yamada originates from the Black Gate Island lineage, the daughter of a renowned genius magician father, Kozo Yamada, who lost his life during a water escape experiment, and a mother, Satomi Yamada, who was once a shaman from the island before becoming a calligrapher in Nagano. This mixed heritage places her at the center of two contrasting traditions: the rational craft of magic inherited from her father and the latent shamanistic abilities suggested by her mother's bloodline. She came to Tokyo with the dream of becoming a great magician, aiming to follow in her father’s footsteps and eventually perform on the grand stage of Las Vegas. In reality, she is a struggling, unemployed magician who lives in a run-down apartment called Ikedaso, constantly behind on rent and hounded by her landlady. She has no bank account, no mobile phone, and virtually no social standing, relying on a landline telephone that rings almost exclusively from calls made by her physicist partner, Jiro Ueda, or occasionally from her mother.

Her personality is a blend of extreme carelessness and keen intellect. She is remarkably clumsy, often tripping, falling into holes, or causing accidents due to her restless curiosity and contrarian nature. Her table manners are poor, and she has an intense, almost compulsive fixation on food, often grabbing or begging for whatever others are eating and claiming she is doing poison tests. She is openly greedy, driven by a desire for money and material gain, and has a flexible sense of ethics that sometimes leads her into questionable schemes or petty crime. Despite these flaws, when confronted with fraudulent psychics or supernatural claims, she undergoes a sharp transformation. Her observational skills become razor-sharp, and she applies the principles of magic tricks to dismantle elaborate deceptions. Her motivation for confronting these frauds is not a strong sense of justice but rather a deep personal connection to her late father, who rejected the existence of supernatural powers and spent his life exposing them. She sees debunking these charlatans as the only bond she has left with him.

In the story, Naoko Yamada serves as the key practical investigator who exposes the mechanics behind each supposed miracle or occult phenomenon. She is often recruited, either by force of circumstance or by the promise of payment and food, by Professor Ueda to accompany him on cases. While Ueda provides theoretical knowledge, logical reasoning, and public credibility, Yamada contributes hands-on magic techniques, keen observation, and a pragmatic understanding of how trickery works. Their partnership is the driving force of the narrative, with Yamada frequently unwilling but indispensable. Her role deepens over the course of the series, particularly in the theatrical films, where she is sometimes asked to impersonate a deity or participate in a battle royale among spiritualists, revealing both her capabilities and the weight of her family legacy.

Her key relationship is with Jiro Ueda, a physics professor who prides himself on being a skeptic. Their dynamic is combative and transactional, with Yamada frequently demanding payment or better food, yet it evolves into a genuine if unspoken partnership. She also maintains a fraught connection with her mother, Satomi, whose latent shamanic abilities and sometimes fraudulent practices Yamada finds difficult to counter logically, often leaving those problems for Ueda to solve. Her deceased father, Kozo, remains a central emotional anchor, shaping her worldview and driving her actions. Other recurring figures include Detective Yabe, a lazy and cowardly police officer who regularly crosses paths with their investigations, and her landlady Haru Ikeda, who is a constant source of pressure over unpaid rent.

Development occurs gradually across the series and films. While her core traits of greed, clumsiness, and obsession with food remain consistent, there are hints of deeper layers. She occasionally exhibits involuntary psychic flashes or healing abilities, suggesting that the shamanistic power from her mother’s side may be real, even though she herself relies on trickery. The conclusion of the series explores her origins and her relationship with Ueda more closely, bringing a sense of closure to her long journey from a struggling magician to a figure who has faced the truth behind her own family mysteries.

Notable abilities include a proficiency in card tricks and zombie ball routines, as she lacks the funds for large-scale illusions. She is capable of improvising simple yet effective deceptions on the spot. When investigating, she demonstrates an exceptional eye for detail and a practical understanding of how light, perception, and mechanics can be manipulated to create false supernatural effects. Though she rarely leads in purely logical or theoretical analysis, her street-smart approach is frequently the key to cracking a case.