WARNING: The following contains spoilers for Episode 12 of The Detective Is Already Dead, "Those Dizzying Three Years I Spent With You," now streaming on Funimation.
The Detective Is Already Dead aka Tantei wa Mou Shindeiru (Tanmoshi) wraps up its first season with the epic return of its titular heroine, Siesta, in a rather unconventional way. Not only does Siesta's consciousness temporarily take control of Nagisa Natsunagi's body to help defeat Chameleon, but Season 1 also neatly ties up every character arc that began with Episode 1. The major revelations in the season finale also potentially set up future storylines for Season 2.
One of the major storylines that comes full circle in the season finale concerns Nagisa Natsunagi and her link to Siesta. Since Episode 2 of the anime, Nagisa has been occupying the role that was formerly held by the deceased ace detective. According to Nagisa's own reasoning, continuing Siesta's work was her way of honoring her legacy and the new life she had been gifted with, thanks to Siesta donating her heart.
Of course, it was only a matter of time before Siesta's former partner, Kimihiko Kimizuka, put two and two together and realized that Nagisa is actually another of Hel's alter egos. Despite being Siesta's sworn enemy, there's a lot Kimihiko doesn't seem to know about Hel, Nagisa and even Siesta herself. For starters, he doesn't seem to know that Siesta played a bigger role in facilitating the Nagisa identity now that her heart is residing within Hel's body; quite a significant reveal.
Sometime after Kimihiko passed out from the fight against Hel a year prior, Siesta revealed that she didn't actually intend to die, but would "wake up" as Nagisa Natsunagi. She also accepts that she is dead and confirms in the present that she doesn't plan to take over Nagisa's body, explaining that she is passing on her legacy to Nagisa, Kimihiko, Yui Saikawa and Charlotte Arisaka Anderson (aka Charl). Her final wish is for the four of them to work together as a team and take out SPES. This is also the condition she gives Nagisa when the latter temporarily grants her permission to take over her body to save Kimihiko from Chameleon.
There's a lot to unpack about Siesta's relationship with Nagisa, beginning with the fact she seems to know more about Nagisa than she lets on. This possibility is particularly strong when taking into account that Siesta didn't want to die, but still decided to accept her fate and pass on her legacy to Nagisa specifically. This is an odd turn of events considering Siesta originally sacrificed herself to save Alicia from her villainous alter ego. It seems to hint that Siesta may have always been aware of the Nagisa identity.
If Nagisa is not a new identity Siesta developed after "merging" with Hel, it's similarly possible that Nagisa may have met Siesta at some point in her respective past. However, as someone who lives with Dissociative Identity Disorder, she's unaware of this. On that note, Hel implied back in Episode 6 that she knows quite a bit about Siesta as well, implying they have a shared history. This seems to further hint at the possible link with Nagisa -- a potential storyline that could be explored in Season 2.
Another major storyline that's wrapped up in the season finale is the unresolved romance between Siesta and Kimihiko, both of whom admitted their feelings for one another on the battlefield a year prior. In conversation, they specifically revisited the time they drank alcohol together in London, resulting in a one-night stand that neither individual is proud of. Kimihiko in particular strongly denies participating in drunken sex with Siesta, most likely as an act of self-preservation given the lack of clear consent.
Siesta neither confirms nor denies the one-night stand, but she does confirm that -- at least once -- she would consider making love with him. This not only confirms Siesta's feelings for Kimihiko are romantic in nature, but also that she's capable of acting on those feelings when her inhibitions are lowered. That's exactly what happened in London the previous year when Siesta -- under the influence -- invited Kimihiko to her bed, facilitating the chain of events that led her to initiate sex. Given the significance of the bedroom scene at this point, it's likely the one-night stand will be revisited in Season 2.