WARNING: The following contains spoilers for Episode 3 of The Detective Is Already Dead, "That's Yui-nya Quality," now streaming on Funimation.
The Detective Is Already Dead Episode 2 revealed the deceased Siesta’s heart is residing within Nagisa Natsunagi, which caused her to experience cellular memory transference. She's even acquired some of Siesta’s personality traits without realizing it. Episode 3 goes further in-depth with this development as we are treated to Nagisa's own insight into her condition -- and the extent of her relationship with Siesta.
Nagisa starts describing herself confidently as a "legendary detective," surprising her sidekick Kimihiko. When asked if she's really intent on working as a detective, Nagisa reveals her "body was already in motion by the time I realized it." What this means is unclear, but it appears to be another symptom of cellular memory transference at work.
Later in Episode 3, Nagisa reveals more about her life experiences before and after her heart transplant. She once again revisits her inquiry from Episode 2 where she felt as though she'd been "searching for someone for a long time" but couldn't figure out who. She then discloses that prior to her heart transplant, she had no real identity of her own due to her disability. Her heart condition resulted in significant physical limitations, impacting the experiences she was able to have as a child, so she reasoned there was no point in having a self-image. This provides significant insight into Nagisa's self-esteem issues that may add another meaning to her "searching for someone for a long time."
One safe takeaway from Nagisa's introspection is that it was neither Siesta nor Kimihiko she was truly looking for, but herself. By learning her heart donor's identity in Episode 2, The Detective Is Already Dead gave Nagisa a new lease on life, even a new purpose. She believed she had a debt to her heart donor that needed to be repaid. This may have been code for "I owe my life to my heart donor, therefore I must make something of myself. I am alive and they're not, therefore I must not waste their gift to me."
This line of reasoning would explain why Nagisa is more confidently declaring herself as a "legendary detective." It's less about becoming someone else and more about honoring Siesta by continuing her life's work. Along the way, Nagisa gets an opportunity to learn about herself and develop a self-concept. While she will continue to have agency and won't lose herself to Siesta, Episode 3 still hints that Siesta could become an important figure in Nagisa's life.
During the walk home from a meeting with their newest client (in this case, the 14-year-old idol, Yui Saikawa), Nagisa revealed for the first time she's been seeing Siesta in her dreams, further developing the idea of cellular memory transference. The way Nagisa describes her experience sounds much like the character we met in Episode 1. She describes Siesta as "pretty," which Nagisa immediately found intimidating. Their interaction, however, seems to validate earlier speculation that Siesta actually intended to cheat death by becoming an organ donor.
While Nagisa doesn't get into details about her argument with Siesta, the fact that they argued at all seems to hint at Siesta wanting to take control of Nagisa's body. At some point, she must have given up and told Nagisa to do with Kimihiko "as she saw fit" with her functioning as detective and Kimihiko continuing to fulfill his sidekick role. The fact Siesta is now starting to appear inside Nagisa's mind has significant implications for their future development.
In fact, two storytelling devices can easily come out of The Detective Is Already Dead's new status quo: either we’ll see future mentorship between Nagisa and Siesta within the confines of Nagisa’s mind, or we might see a situation where Nagisa gives Siesta permission to "pilot" her body in situations that would require her expertise.
