Nagisa Natsunagi (夏凪 渚 Natsunagi Nagisa?) is the secondary main heroine and one of the main characters of The Detective is Already Dead. series. She is an 18-year-old high school student, who was the first client to request Kimihiko Kimizuka as a detective since after Siesta's death, to find someone.
Nagisa is an 18-year-old high school girl, considered to be beautiful by Kimihiko, with hip-length black hair that is tied in a short tail where it is pulled to the right side and it is tied by a red ribbon with bangs hanging over her forehead while she has ombre tips and red-brown eyes.
Nagisa has a strong personality, but she is a dominant character whose habit is to say "Die Twice".
As a child, Nagisa had a heart condition while she also had a weak and fragile body which forbid her from going outside and to school meaning she spent most of her childhood in a ward room's hospital bed. One year before meeting Kimihiko Kimizuka, Nagisa received a heart transplant from an unknown donor that later was revealed Siesta was the donor. She inherited one of Siesta's "Seven Tools", the musket gun.
- Nagisa means "beach, shore, water's edge" (渚?)
- Natsunagi means "summer" (夏?) (natsu) and "calm, lull" (凪?) (nagi).
- According to Nigozyu, Nagisa was intended to be the only main heroine in the manuscript submitted for the Rookie of the Year Award.