To tell the story of Death Note, its creators made 37 episodes, 12 manga volumes, multiple live-action adaptations, and a few light novels. With all this content flying around, it makes sense that Death Note obsessives have come up with all sorts of clever fan theories. An anime that involves mind games, mystery, and the concept of morality, Death Note practically creates its own conspiracies from the get-go. That's why so many people consider it their favorite anime of all time, and turn to more shows like Death Note.
Death Note fan theories help make sense of the anime's deep questions and the loose ends it fails to close. Like Attack on Titan fan theories, Death Note theories add new layers to the series while also offering conclusions to its open endings.
In Death Note: Relight, an unnamed Shinigami appears. Many fans believe this Death God is Light reincarnated. Plenty of evidence exists to support this theory. For one thing, the Shinigami wears a blood-stained coat resembling the one Light dies in.
Additionally, his mouth and voice appear similar to Light's. He knows about Ryuk's unusual affinity for apples, even throwing one to Ryuk the way Light did in the past. There are continuity issues between the original Death Note and Relight, so it's possible this Shinigami is meant to be Light, even though the original series states nothing happens after death.
Over the years, the Death Note fandom has diagnosed various characters with certain medical and mental ailments. These are the most popular ones:
L Has Marfan Syndrome: Marfan syndrome is a genetic disorder with a variety of symptoms, including spidery hands and scoliosis, a sideways curvature of the spine. It also causes threatening issues in the lungs, heart, and nervous system. Though he never suffers debilitating health issues in life (as far as we know), L does have a curved spine and long, spider-like fingers.
Light Has Antisocial Personality Disorder: Antisocial Personality Disorder, sometimes called sociopathy, is a "mental condition in which a person consistently shows no regard for right and wrong and ignores the rights and feelings of others." Light exhibits many of the symptoms of APD, though he was generally well-behaved and showed some empathy until power dramatically warped his personality.
Near And L Are Autistic: According to the Autism Self Advocacy Network, "Autism is a neurological variation that occurs in about one percent of the population and is classified as a developmental disability." It makes typical modes of social interaction difficult for patients. Autistic people may exhibit atypical and repetitive movements, a deep focus on a narrow range of special interests and non-standard ways of solving problems. Some people believe that L and Near approach their interpersonal relationships and their detective work in ways consistent with the autism spectrum.
Near Has Albinism: Near sports white hair, a trait that wouldn't be remarkable if he existed in a different anime. But Death Note has a fairly realistic art style. Some fans hypothesize Near's hair is the result of albinism, a group of disorders in which the body produces little-to-no melanin.
L starts working as a detective at the tender age of eight. The start of his career remains unclear; all we know is that he was an orphan and that this job likely wasn't his choice, but Watari's. As he gains experience, he learns the importance of approaching cases with little-to-no emotional involvement - in matters of life and death, getting too involved often results in self destruction.
This, according to Redditor u/Azmek, is why L lies to the orphans at Wammy's House about his motivation for solving cases. In the Death Note one-shot special, L tells his potential replacements he works for the thrill of solving puzzles, not out of a particular desire to help anyone. But L, likely to his own chagrin, never fully disengages from his emotions. He develops a genuine friendship with Light over the course of the Kira case, changes his unethical investigation methods when his team objects, and feels genuine sorrow when Ukita, one of the task force members, is killed.
L knows his feelings hinder his progress on the job. What he tells his orphan admirers is, at most, a half-truth. He knows detachment will lead to victory, but L cannot convey to them that he's incapable of it himself. Near's lack of emotional attachment is partly of why he catches Kira, while L failed.
According to DuoReview, Near actually uses the Death Note to kill Teru Mikami. Mikami's sudden suicide could easily be explained by the fact that his god, Light Yagami, breaks down before his eyes. Mikami is also facing at least life in prison, possibly the death penalty. Yet the sudden and dramatic way he ends himself is not only jarring, it goes against Mikami's uptight nature. So, was he manipulated into suicide by a Death Note?
In the final episode, Near reveals he held the notebook on his person for quite some time, so he certainly had the means. Mikami's death makes wrapping up the case easier, and, as DuoReview argues, Near's apathy would likely allow him to do such a thing.