Found some old and unfinished fic, Kai perspective. Ostensibly Kei/Kai but Kotobuki shows up. Warning for sad (at least when I read it). I can't vouch for it having plot but I figured I'd share anyway.
Under a read more for length. (I say that but it's only ~800 words)
In prison there wasn’t much to distract Kai from thinking about Kei. He had thought about trying to keep track of the number of times Kei came to mind, as a way to pass the time, but they had taken away his phone (the new one), and the prison uniforms didn’t have so much as a pocket for keeping pencil and paper. He had considered marking tallies on his body, but gore wasn’t really his thing.
He ate his food slowly, methodically, staring into space. How many times had he reviewed the memories? It didn’t take long. It was a few short days, the time they had spent together, the one epic night of escape and the distant warmth of their childhood summer. Kai wondered what Kei was thinking about. He knew it wasn’t him. He hoped Kei was safe, at least- that no news was good news. But it had been six weeks since their run from the police, and three weeks since Kai and been tried and locked up. The last he had heard, Kei had dropped a researcher off the government building and then careened into the river. The only relief was the knowledge that Kei wasn’t dead, at least. Kai could never lose him that way. As long Kai stayed alive, Kei would be somewhere out in the world, and hope, if that’s what you could call it, remained.
Kai didn’t really understand Kei’s actions, about dropping the researcher and jumping into the river, but he had been hearing about Satou, the rogue Ajin who had attacked the Grant pharmaceutical company. Kai bristled. Ajin was the right word for that one. Nothing Satou had done so far proved that he was, or had ever been, human.
A boy sat down across from him at the table, tray smacking the surface, soup slopping over.
“Hey. Whatcha looking at, punk?”
Kai showed no sign that he had heard, but he could feel the instinct to lash out rise up in his chest.
“You’re that kid who ran away with the Ajin, arentcha? Nice little reward you gotchyourself here, stuck in prison. When’s he gonna come get you out, huh?”
Kai closed his eyes, trying to think only of Kei, but his instincts wouldn’t leave him.
He waited for the other boy to continue his attack, but the banter stopped. He opened his eyes. He was sitting silently now, just staring at Kai.
“You’re no fun.” Inmate number 16005 turned back to his tray, and started eating.
Kai closed his eyes again. The anger still seethed as he pushed away the thought of serving out the rest of his sentence here. He was lucky that guy on the motorbike was still alive, so he only had been served 3-5 years for various counts of assault.
To calm himself down, Kai ran through his memories of Kei again, this time pausing longer to consider each one. First, that fateful message from Eriko and the dull dizzying pain of losing his first friend, combined with the dread of going back to the bleak everyday before. Then he flashed back to meeting Kei for the first time, and the delicious jumpy feeling of realizing he a friend, a friend, his very own! That excitement smoothed into the recent sureness of speeding towards the forest after Kei’s call, knowing that now was his moment, Kei’s panicked voice on the phone serving only to calm Kai’s sure decisiveness that he would rescue Kei. The crashing of bodies and branches as he tumbled into the police officer; the wind of the bike again, this time with Kei safe behind him, like a princess being whisked away on a steel chariot. How his body had frozen up at Kei’s voice, the Ajin voice, but released, into pain from the attacks and more wind, the graze of the railroad warning bars just over his head, and sooner than he wanted the awful swerving of his motorcycle, then sailing through the air and smashing onto the riverbank. Here he concluded every time that a miracle had occurred, and that if he had lived through that then saving Kei must be his destiny.
The next part was hard to think about. Looking back, he went from seeing his friend broken and in pain, to bleeding and dead, and back to normal in almost the same instant. Those images would never leave his head, as much as he tried to forget them. The power of an Ajin was truly something terrifying. Kai unwittingly remembered how for a second his faith in Kei’s humanity had faltered, but he had been over this with himself before: Kei had been and always would be just as human as any other human Kai had had the misfortune to know. Even if no one else could see that, no one else had seen Kai either, except that one boy on the playground who had come over and asked if he wanted to look for bugs.
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The following is related only in the sense that it's also Ajin fic and it was in the same document as the above (ostensibly Kei perspective):
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I am a human, right? Raised by humans, I look like humans... but what in the world is a human, anyway? Humans are fragile creatures, can't seem to do right even when they have a hundred chances. But... they don't stop trying. Weak as they might be, that pilot light way deep down, never goes out, and there's always a chance for a new start.
Ajin are an expression of that, taken to the extreme. Someone, somewhere, knew that human bodies weren't made to contain human souls. Pathetically limited shapes of flesh, only good for blending in with the rest of the sad flesh shapes. They celebrated themselves, but what was there to celebrate? It was so easy to rearrange them, even when the mind inside had hardened beyond repair.
