WARNING: The following contains spoilers for The Detective Is Already Dead, now streaming on Funimation.
Dissociative Identity Disorder (DID) -- formerly known as Multiple Personality Disorder -- is characterized by the presence of more than one personality within the same body. One of the symptoms experienced by individuals with DID is memory gaps surrounding specific events and even specific relationships. Other symptoms people with DID experience include feeling out of touch with their own emotions, not having a real sense of identity and even having out-of-body experiences. Being such a complex mental condition, one wouldn't expect an anime to get much about it right.
However, The Detective Is Already Dead (aka Tanmoshi) based on the light novel series by Nigojū, offers a surprisingly thoughtful portrayal of DID through one of its main characters, Nagisa Natsunagi. Nagisa is not only depicted with common DID symptoms but is also portrayed with life experiences that are consistent with those lived in real life by many people with DID.
Nagisa Developed Her First Alter Ego In Response To Trauma
While current research hasn't found a definitive cause for DID, one recurring pattern among those with DID in real life is that many experienced extreme trauma in childhood. This is explored with Nagisa Natsunagi in the anime and in much more in detail in the light novels.
In both the first season of Tanmoshi and the first two light novels, the terrorist organization SPES abducts and conducts experiments on orphan children. One child that was experimented on to the point of torture was a 7-year-old girl named Alicia. Unable to process the trauma she was experiencing, Alicia developed her first alter ego -- the villainous personality Hel -- most likely to protect her from the abuse she was enduring. Hel, however, ended up forming an alliance with Alicia's abuser, which got the attention of Siesta, an ace detective who ended up "merging" with Hel to subdue their villainous consciousness. This led to a new personality, Nagisa Natsunagi, emerging.
Nagisa's Alter Egos Are Different Ages and Genders
Another thing Tanmoshi gets right in its portrayal of DID is that Nagisa's alter egos also tend to differ in age and gender. Nagisa herself identifies as an 18-year-old high school girl with a serious heart condition that required a heart transplant. Because her disability severely limited her ability to make friends and have a social life, Nagisa suffers from low self-esteem and obsesses over making herself useful. She believes she owes it to her heart donor to make something of her life and decides to take up the occupation of her heart's former owner.
Nagisa's other alter ego, Alicia, is an amnesiac 7-year-old girl who believes herself to be 10 years older than she actually is. She knows nothing about herself, where she comes from or if she even has a family. She does, however, develop a strong liking for Siesta and her partner, Kimihiko Kimizuka, after they show her kindness and allow her to stay with them. Alicia came to see Siesta as something of a mother figure and developed a child-like crush on Kimihiko. When Siesta was recovering from an injury, she allowed Alicia to fill in for her as a substitute detective, much to Alicia's delight.
However, the villainous alter Hel is a more radically different personality, beginning with the fact they identify as non-binary. Not only does Hel dress like a man and use male pronouns in Japanese, but their behavior is also noticeably masculine. In contrast with both Nagisa and Alicia who struggle with feelings of low self-worth, Hel is much more confident. They take pride in being SEED's "right-hand man" and is entirely driven by a desire to win SEED's approval. While they are reluctant to admit it, Hel wants to be loved and valued by someone important.
Nagisa Experiences Gaps In Her Memory
Another symptom of DID TanMoshi correctly explores with Nagisa is the fact that there are no shared memories between herself and her alter egos. Nagisa herself doesn't actually know how she got her new heart, as it was Hel who murdered Siesta and took her heart. Those memories aren't Nagisa's. Instead, her mind developed a backstory of having a heart condition that required a transplant.
Alicia similarly does not remember anything that happens whenever Hel takes over her body. This was especially true in London, when Siesta impaled Hel in the heart with their own sword during their first fight. Whenever Alicia fell asleep, this was when the Hel personality would seemingly awaken to murder victims for their hearts. Alicia only became aware that this was happening after Siesta put two and two together and realized Alicia has DID.
Nagisa Is Aware of the Other Alter Egos
Of the three alter egos, only Hel seems to be aware of Alicia's existence from the start. They are especially aware of the abuse Alicia experienced from SEED to the point where Hel similarly abuses the girl, revealing a more sadistic nature. Hel is also capable of dissociating at will and allowing Alicia to surface when their life is in immediate danger, which is an abusive act in itself. Alicia doesn't become aware of Hel's existence until Siesta makes her aware of it, and she soon comes to fear hurting the people she loves when she's not herself.
After Hel murders Siesta and transfers her heart into their own body, the Nagisa personality that emerges afterward is only aware of Siesta residing in her head. She only interacts with Siesta in her sleep but is also capable of dissociating at will and allowing Siesta to take control of her body when her expertise is needed. It isn't until after her first encounter with Chameleon on a cruise ship that Nagisa becomes aware of her Hel alter ego and the truth about Siesta's fate.
Nagisa Feels Out of Touch With Herself
Before Nagisa becomes aware of her DID, she persistently feels she has no real identity as a person. At first, she thinks this is due to her physical disability and the fact she wasn't able to make friends during childhood. As such, she didn't see any point in developing an identity prior to obtaining Siesta's heart.
Even after Nagisa committed herself to continue Siesta's work as a way of honoring her, she continued to struggle with an identity crisis. As she states in Vol. 1 of the light novel series, she wants someone to validate her as an ace detective in order to be able to live up to Siesta's legacy, both in name and in fact. The person she wants this validation from more than anything is Siesta herself, especially since Siesta is more logical than intuitive, whereas Nagisa is the polar opposite of that.
While series creator Nigojū does further develop the circumstances of Nagisa's DID in the remaining volumes of Tanmoshi, in just the first two volumes alone, which also make up the first season of the anime, he still accurately portrays common symptoms of the disorder, along with the life experiences that facilitate it.