Natsumi Kousaka is a twenty-seven-year-old Japanese woman who owns a confectionery shop in Paris, where she has lived for seven years. She was born on May 3 and has dark brown hair and distinctive grey eyes, a trait she inherited from her great‑grandmother. Her family history is deeply connected to the Russian imperial court: her great‑grandfather, Kiichi Kosaka, was a craftsman at the Fabergé factory, and her great‑grandmother was Maria, a daughter of Tsar Nicholas II. Kiichi and Maria eloped to Japan during the Russian Revolution. Natsumi’s parents and grandfather died in a traffic accident when she was five, and she was raised by her grandmother until the grandmother’s death about a month before the events of the film. Returning to Japan to settle the estate, Natsumi discovers a fragment of a sketch showing two Fabergé eggs and a distinctive key among her grandmother’s belongings. This sketch sets her on a path to uncover a hidden second egg that her great‑grandfather had made. She is motivated by a desire to understand her family’s past and to preserve its heritage. In the story, Natsumi meets Conan Edogawa, Ran Mouri, and Kogoro Mouri while traveling by ship to Tokyo. She shares her knowledge of the eggs and the key, becoming an important link in solving the mystery surrounding the Fabergé eggs and the murders committed by the killer known as Scorpion. Her key relationship is with the family butler, Kuranosuke Sawabe, who accompanies her to Yokosuka Castle, the site of the hidden egg. Natsumi is portrayed as earnest and somewhat reserved, yet cooperative and curious. She does not display any special combat or detective abilities; her primary contribution is her familial knowledge and the key that opens the coffin containing the second egg. During the climactic events at the castle, she helps Conan and the others navigate the secret passages, and she is present when the truth about her lineage is revealed: she is a descendant of the Romanov dynasty. By the film’s end, although the castle burns down, Natsumi is satisfied that the egg and her great‑grandmother’s remains are safe, and she accepts the historical significance of her heritage. Her character arc centers on reconciling her personal loss with the discovery of her extraordinary ancestry.