Natsumi Miyadai is a supporting character in the nineteenth Detective Conan film, serving as one of the seven specialists known as the Seven Samurai, who are assembled by Jirokichi Suzuki to protect Vincent van Gogh's Sunflowers paintings during a major exhibition. She is a painting history professional and a painting appraiser, responsible for authenticating the artworks and ensuring their historical accuracy. Her expertise is highly regarded, and she is described as having studied all theories surrounding van Gogh's Sunflowers series, demonstrating a deep and scholarly devotion to the artist's work.

Her background establishes her as a serious academic deeply invested in the integrity of art. She is presented as a competent and dedicated professional, appearing reliable and trustworthy as part of the exhibition team. However, beneath this composed exterior lies a fanatical obsession with van Gogh's paintings. She holds the belief that two of the Sunflowers paintings intended for the exhibition are forgeries. To her, the idea of displaying what she considers fake works alongside genuine masterpieces is an unforgivable desecration of van Gogh's legacy. This conviction drives her entire role in the story.

Her motivation is not theft or personal gain but destruction. Convinced that the two paintings are not authentic, she becomes the true mastermind behind a series of dangerous crimes intended to incinerate them. She believes that it is better for the forgeries to be completely destroyed than to be allowed to stand in the company of the real paintings. This extreme, self-righteous belief fuels her actions, making her a villain driven by a twisted form of artistic purity.

In the story, she initially appears as a helpful and knowledgeable ally. She is the one who discovers a Kaitou Kid card on one of the paintings and requests to take it to her studio for a more thorough examination, a move that is part of her plan to isolate and destroy the work. She is also the one who suggests filling the museum tunnels with artificial sunflowers as a camouflage measure against security cameras, a suggestion that later reveals itself as a key part of her plot to use the flowers as kindling for a fire. Her role is that of the hidden traitor among the Seven Samurai, the Judas figure that Kid's warning alludes to. Throughout the film, she subtly manipulates events to create opportunities for her destructive plans, orchestrating the bombing of the cargo plane and the subsequent fire at the Lake Rock museum. Her plan is ultimately exposed by Conan, who reveals her as the true culprit behind the attacks that were blamed on Kaitou Kid.

Key relationships center on her fellow members of the Seven Samurai, whom she deceives, and the target of her obsession, the Sunflowers paintings themselves. She also has an adversarial relationship with both Conan Edogawa and Kaitou Kid, as they actively work to thwart her plans, with Kid even pretending to steal the paintings in order to keep them safe from her.

Her development in the film is minimal, as she remains steadfast in her convictions until the end. She does not experience a change of heart; rather, her schemes are forcibly stopped by the protagonists. Her notable abilities lie in her field of expertise: she is a highly skilled painting historian and appraiser with a profound knowledge of van Gogh. It is this very expertise that, combined with her obsessive fanaticism, allows her to almost succeed in her mission of artistic arson. Her character serves as a dark example of how scholarly passion can warp into destructive extremism.