Mikami is L
Rem was (understandably) pissed about being made to sacrifice herself. So, she writes in her Death Note, "L Lawliet. Suicide. Apparently dies of a heart attack, escapes from his grave with his memory destroyed, adopts an alias, starts a new life and becomes obsessed with Kira. He is devoted enough to gain Kira's trust, but fails at a critical jucture, causing Kira's defeat. He commits suicide out of shame afterwards." However, Rem filled in few details of L/Mikami's behavior post-amnesia other than his obsession with Kira, leading to a methodical, obsessively followed schedule.- The Death Note can only control people for about a month, though. He would have had to come back from the dead mighty quick... which presents an amusing image.
Mikami was followed by a Shinigami for large periods of his childhood
During Mikami's time in high school, the 4 bullies and his mother were all killed in the same accident, as well as the "coincidental" times when "he wished for a deletion, it would come". All this points to a sympathetic Shinigami's involvement. Furthermore, none of these killings would have extended Mikami's lifespan, allowing the Shinigami to remain alive.- This seems plausible.
- The manga says "it was all a coincidence". This isn't a character saying this, it's the actual narration. So it must be coincidence.
- A real coincidence would be if Ryuk was the Shinigami following Mikami around. Think about it. He was probably getting bored even then, and so he might've decided to see if he could convince some human kid that he had magic killing powers just to mess with him. Mikami ended up being way too boring, and so Ryuk would've left after a while, coming up with his 'Throw a Death Note into the Human World' plan afterwards as a much more interesting idea with less work on his part.
- A rule in the Death Note completely disproves this:The god of death must not stay in the human world without a particular reason. Conditions to stay in the human world are as follows:
When the god of death's Death Note is handed to a human.
Essentially, finding a human to pass on the Death Note should be done from the world of the gods of death, but if it is within 82 hours this may also be done in the human world.
When a god of death stalks an individual with an intention to kill them, as long as it is within 82 hours of haunting them, the god of death may stay in the human world.- It's true that a Shinigami couldn't follow Mikami per se, but it could still watch him from the Shinigami world and kill the people he wanted to die.
- Well, if say Ryuk was the Shinigami to kill all the people that Mikami wanted to delete, then technically he does have a reason to be in the human world. He's stalking all those people Mikami want dead to kill them, which falls into perfect line with the rules. He could have first came into the world, stalking Mikami's mother to kill her. After writing Mikami's mother's name in the Death Note, he noticed Mikami's "delete" issue, so decided to add the bullies in the Death Note so that they all died in the same accident. After that, he followed Mikami to get names of people he could stalk to kill in the Death Note. As he has 82 hours per life he's taking, taking many lives gives him 82 hours longer that he could spend in the human world each time he kills. At some point, either he was killing too much or just got board as said above, so returned to the Shinigami world.
Alternatively, Mikami was friends with someone who secretly owned a Death Note at the time.
It's not unreasonable that it could have happened. It is possible that after suffering a really bruetal beatting from the four bullies and having to hear his mothers thoughts on the matter which went against everything he stood for, he could have mentioned to his friend shortly after in a despairical rant about how the bullies and his mother should just be 'deleted', to which the Death Note owner was more than willing to oblige to... Mikami had a Death Note before the start of the series
Most Shinigami don't seem to care much for the human world beyond increasing their own lifespan (aside from Rem, Gelus, and Ryuk - Shido doesn't count, since he's only there to retrieve his note). So, to explain all the coincidental deaths and deletions around Mikami, he used to have his own Note, which he gave up at some point. Taro Kagami from the pilot chapter grows up to become Teru Mikami.
After the incident in the pilot he changes his name when he's like put into witness protection or something. He forfeits ownership, loses his memories, and later regains them at some point because Ryuk thought it would be funny. He then goes on to write the story of Death Note once the world has fallen under Kira's control for the sake of plausible deniability.- Also, he still has the Death Eraser and eventually brings L back to life on Light's request because Victory Is Boring.
- Maybe all of those deaths he saw as a child was actually things he wrote in the death note and then he gave up his death note losing memories but the deaths were mentioned so Taro remembered. The end picture with Taro and Ryuk could have been when Light gave Teru the notebook but Ryuk gave it to Taro instead. Taro killed Mikami and replaced him with himself. He became methodical because Taro was afraid that any change in his behavior would make it apparent that he was an imposter.
Mikami pulled a Heel–Face Turn before he died
After Light said "I Don't Know This Guy" in the last episode, Teru Mikami finally realised that he had been used, and saw how evil Light had become. Upon realising that he'd dedicated his life to serving someone who, in his mind should be DELETED, he broke down and killed himself Mikami and Light are Childhood Friends.
But they lost touch after Mikami's mother died and Mikami was shuffled off to some orphanage and went to a different school. That would explain why Light chose him so quickly when he saw him at the Kira Rally. Light could have even been one of the kids that Mikami saved from bullying! Mikami is host to a Dark Passenger
Think about it. Light is motivated by a God complex and a desire to change the world, Misa appears to be motivated by love for Light, and Higuchi is more or less motivated by Greed. (Wait, I'm seeing a bit of a theme here...) Mikami, on the other hand, appears to just really enjoy deleting for its own sake. A Dark Passenger is, more or less, Killing Intent. Mikami either has failed at keeping his Dark Passenger at bay or has decided to embrace it.