Detective Is Already Dead begins to have multiple endings in its penultimate episode, starting with the truth about Nagisa’s Natsunagi identity.
The Detective Is Already Dead aka Tantei wa Mou Shindeiru is looking to end the first season on a mass basis. In episode 11, loose ends begin to come together as Kimihiko Kimizuka begins to piece together what really happened to his life partner, Siesta, on the fateful day he lost her. During the brutal fight against the SPES opener Chameleon, Kimihiko begins to recall her last conversation with Siesta on the battlefield a year ago.
After Hel succeeded in stealing Siesta’s heart and placing it in their bodies, a grieving Kimihiko attempted to attack her killer. He quickly stopped when he heard the familiar voice of his deceased mate emanating from the lips of their sworn enemy. After taking control of Hel’s body, Siesta communicated her game plan to Kimihiko from the start.
Before meeting Kimihiko on the plane three years ago, Siesta knew about Hel and Alicia. She also knows that in order to stop SPES and save Alicia, Siesta will have to find a way to stop Alicia’s more villainous alter ego. She could very easily kill Hel if she wanted to, but she knows that killing Hel also means killing Alicia. After going through the various plans in his head, Siesta finally concluded that the only way to truly stop Hel and save Alicia was to allow Hel to kill her and take her heart.
Since Siesta’s heart was specifically designed to contain her consciousness, this would allow her to penetrate Hel’s body and subdue their personalities. Of course, Siesta didn’t inform Kimihiko of this before working with him, which understandably annoyed him. Having loved Siesta up to this point, Kimihiko felt it was cruel of her to make important decisions about her life on her own. Naturally, Siesta knew he would try to stop her from completing the quest if he knew all of her plans.
Despite the deceptive nature that Siesta performed in defeating Hel, she tearfully confessed her feelings for Kimihiko by promising to find him again in the future. At this point, pollen from nearby flowers that have the ability to erase memories has flown into the air. Fearing memory loss, Kimihiko tearfully confesses to Siesta that he doesn’t want to forget her, and she reassures him that he won’t. He will forget some things, but not her.
In the present, Kimihiko begins to pair the two together and realizes that Nagisa Natsunagi is actually Hel – with a new personality. His suspicions are soon confirmed when Siesta – in Nagisa’s body – rescues him from Chameleon by shooting him with the same blood bullet she shot Bat in the back in Episode 1. she did with Hel a year ago, Siesta took control of Nagisa’s Body, leading to the reunion she promised Kimihiko.
Given The Detective Is Already Dead’s explicit confirmation that Nagisa and Hel are one and the same, and that Siesta can “control” her body at any time, this has significant implications for the origin of the story. Siesta herself. Considering how Alicia was tested to the point where she ended up with a body that could literally change her appearance, it’s very possible that Siesta herself was tested by SPES as well. This would certainly explain why Siesta knows so much about the organization – and why she has a custom heart that Hel could easily implant into her own body without rejecting it.