Natsuko Ooshima (大嶋 夏子, Ooshima Natsuko) was a rookie detective who was part of the Sakurami City Police Department's Investigation Division. She was killed by Minene Uryu so that she can assume the rookie's identity and infiltrate Sakurami Police Station undetected.
Not much is known as Natsuko as a character, but she does bare a striking resemblance to Minene Uryu, sporting the same purple hair and eye colour. However, at least through Minene's portrayal, she is shown to be very cheerful and upbeat. Minene continued to act out Natsuko as being very giddy at the fact that Minene had sent a warning of her next target. It seemed that Masumi Nishijima was rather fond of Natsuko, falling for Minene's masquerade and even asked "Natsuko" out on a date. Nishijima was emotionally devastated upon learning of Natsuko's death, although his feelings for her seemed to change his view of Minene over time. Natsuko may have been smarter than she appeared, possibly digesting her name tag so she could be identified when her body was found to reveal Minene's crimes.
Future Diary: Mosaic
In Mosaic, Natsuko was a rookie police detective under the command of Nishijima. However, whilst on a scuba diving trip, Natusko was ambushed by Minene and murdered; trapped in an oil drum which sank to the bottom of the sea. Minene took on Natsuko's identity and fooled the police by perfectly mimicking Natusko. This was all part of her plan to eliminate Cardinal Fred Haiman in her ongoing terrorism against all things religious.
Whilst Minene paraded around as Natsuko and her attempts to assassinate Cardinal Haiman were unintentionally thwarted by Nishijima's constant appearances. Nishijima even asked "Natsuko" to go on a date with him, but Minene's own hatred for God briefly surfaced. Eventually, Nishijima learnt of the discovery of Natsuko's body by some coastal cleaners. Showing Kurusu the evidence, Nishijima reveals a name tag found in Natsuko's stomach, likely to reveal her fate.
The next day, as Minene activates a bomb to kill Haiman, Nishijima raises his gun to her, revealing he is aware of her deceit. Kurusu appears, revealing Minene was fooled to expose her lies. Seeing Nishijima was hurt by her deception, Minene strips off her disguise and makes her getaway on a high-wire, only for Kurusu to shoot the wire. Minene falls but is saved by Deus Ex Machina and Muru Muru to participate in the Survival Game. There she gets her diary transformed by Muru Muru into her Escape Diary.
After The Final Battle, though not explicitly shown, it is assumed Natsuko is alive in the third world.
- Natsuko's character arc is not covered in the anime; Minene only appearing as Natsuko in Sign Up for one scene.