|Japanese name:|| 越水 七槻|
|English name:||Natsuki Koshimizu|
|Age:||20 (Claimed to be 18)|
|Nicknames:||Detective of the South|
|First appearance:|| Manga: Chapter 562|
Anime: Episode 479
|Appearances:|| TV: 1|
|Keyhole number:||Volume 54|
|Japanese voice:||Risa Hayamizu|
Koshimizu was a High School Detective from Fukuoka, the capital of the Fukuoka Prefecture on Kyūshū, the southernmost island of the Japanese mainland. It is not known for how long exactly she has been engaged in investigative work; according to her own statement, however, she was a relative newcomer, with only 100 solved cases on her credit. However it unaware how much of the background she gave at the introduction was made up to create her cover.
Natsuki is apparently a capable investigator; however, as the Detective Koshien Case has proven, she tends to be extremely stubborn and single-minded if a case touches her personally, and she pursues the culprit(s) relentlessly until they are brought to justice. Though she is by no means a thoroughly evil person, but actually a gentle and considerate character, her persistance does tend to cloud her judgement at times. On her plus side, she is a careful planner and strategist, as well as being a natural actress.
She is willing to let herself and the involved people in the lavender case die along with innocent people to complete her revenge, so she does express regret of dooming Conan to his death, suggesting that she is not completely ruthless.
 Plot overview
One year before Natsuki's first appearance, and a year after she had finished high school, a wealthy socialite was found hanged inside her manor house. The police and Natsuki concluded at first that she must have committed suicide (as she really had), but some time later Junya Tokitsu, another high school detective, appeared on the scene and declared that her death was actually a murder. This was a serious misconception, as Junya had fallen for a trick prepared by a burglar after the murder, but it was enough for the police to open a murder investigation. The only suspect in the case was the lady's young housemaid, Natsuki's best friend. She was repeatedly and harshly questioned by the police, and even though no conclusive evidence could be brought up against her, the girl cracked under the strain and eventually committed suicide.
Because Junya probably later realized that his deduction had been false, he asked to remain anonymous, and so Natsuki had only few scant clues to work with as she set out to avenge her friend's death. She finally decided to arrange a fake detectives' reality TV competition (Detective Koushien), with the most reputable High School Detectives (who also had unique speech pattern traits) attending; next to Junya and herself, this included Heiji Hattori and Shinichi Kudo (Although he was never actually on the invited list in the first place and could attend only as Conan Edogawa because of Heiji's request to bring him along) and Saguru Hakuba, who had been asked to fill in for Shinichi,which really wasn't the case. She only made it appear that Shinichi had been invited, due to his popularity, but couldn't be contacted, as a excuse to why Hakuba was invited instead. She arranged for her trap to be sprung on a remote island, cutting off all connections to the mainland while she sought to find the high school detective, who was involved in the Lavendel Manor case.
Natsuki finally worked out that Junya was the person responsible and killed him by bashing his head with a hammer when he demonstrated his callousness and self-interest about the past case with no feelings of remorse whatsoever. She also trapped herself and her entourage on the island to die of starvation, since two of them - the burglar who had unintentionally backed up Junya's claims; and the manor's butler, who had refused to clear the case out of misguided affection for the lady, since he didn't want people to know she was mentally ill (due to the huge prejudices against mental illnesses in Japan) - were also (if more indirectly) responsible for her friend's death. Conan and Heiji managed to expose her as the culprit, and she quietly allowed herself to be given to the authorities, but not before expressing her deepest respect for the two and the hope of working with them some time in the future.
- Natsuki had proven so popular that the idea for a video game with her as a protagonist was proposed. While Aoyama refused, since Natsuki was a criminal, she became the template for another tomboy detective girl who would be integrated later in the series: Masumi Sera.
 In other languages
|Language||Given Name||Family Name||Notes|
|Japanese||七槻 Natsuki||越水 Koshimizu|
- ^ a b Case Closed Volume 55 - File 2
- ^ Manga Chapter 562: Detective Koshien Case
- ^ Manga Chapter 564: Detective Koshien Case
- ^ Manga Chapter 566: Detective Koshien Case
- ^ Manga Chapter 566: Detective Koshien Case