NOTE: Watched season 1 (and 2 in 2017)
Series in one sentence:
Follow the life of a kid you're not sure you should be rooting for.
Series in more sentences:
In a world where ghost-spawning, immortal humans exist, called 
"ajins", and are globally hunted and subjected to cruel government 
experiments, a boy named Kei gets hit by a truck one day and discovers 
he is one.
His survival gets immediate media attention and he is forced to leave
 his old life behind. Meanwhile, a group of ajin vigilantes plan 
retaliation for the mistreatment their kind has suffered.
Upon sitting down for this anime, I was immediately disappointed by the 
animation quality. It's this cell-shaded 3D animation that moves, like, 5
 frames per second. A kickstarted solo animator at Youtube could've done
 this.
 
But as you can see, I gave this series a good rating. In fact, this is one of my all-time favourite shows. 
The story, the atmosphere, and the concept of the ajins alone are awesome. 
Nevertheless, something I tripped over in my first viewing was the 
portrayal of the relationships between the characters. The characters 
themselves started off fine, as characters tend to when you're first 
introduced to them, but after I finished watching season 1, I decided to
 sit down and think about what exactly made me feel this way. 
There were no charming friendships, connections, or interactions in 
AJIN. It all felt a bit bare, fake, or outright hostile. Everything in 
this fictional world felt dreary and loveless. There was always this 
palpable distance between the characters, even when they teamed up 
together. Not even the two friends who're described to be friends really
 felt like friends. Maybe that's because their friendship had already 
ended, but if you push them back together for this story, why do their 
interactions still feel so rehearsed?
I was also surprised that none of the people who chased down Kei showed a
 second of concern for the fact he's just a child, also unfamiliar with 
his abilities, nor can I recall anyone ask themselves the question why 
humanity is judgemental of these beings at all. 
And as for the main character himself.. that guy is the most difficult 
to understand, even though the series explains word for word what his 
deal is. But just saying something is so is not enough. 
Kei's actions and reactions were all over the place in season 1 and only
 sometimes matched the given claim that he is a selfish dick. He's 
described to follow one life rule: that only the things that directly 
affect him matter, but I found it to be inconsistent.
He starts off pretty calm and boring, like you're dealing with this 
mousy intellectual who has nothing going on in his life and is happy 
with the bare minimum, but when he gets chased around with his friend, 
he suddenly has nothing but angry thoughts. Then, when he gets captured,
 it's all tears and lamenting like a fragile soul. 
One second he saves strangers without giving it a thought, and the next he's arguing with himself that they're not worth it.
  
On paper he makes for an interesting character, but not one anyone would
 feel sorry for, like the series tries to do at certain points. I simply
 don't like him enough to care whether or not he gets captured again. 
For the sake of the story I do, but not for the character that is Kei.
After escaping the dreaded operation chamber,  he grew way too 
overconfident too quickly for my taste, making it hard to sympathize. 
Even the villain commented on it, but like I said before, "saying 
something is so is not enough". 
Is Kei supposed to be an innocent thinker, able to smile and shed tears 
for himself and others, or this uncaring stone-faced sociopath the 
series mainly wants him to be? Is he playing the people around him with 
fake concern? That can't be it, he truly seems to care about those he 
announces to care about, but at the same time, he was quick to abandon 
his best friend when they were still children. So, what is the extend of
 his ability to care? Some people might answer "he cares about others if
 it benefits him", but I'm not yet convinced of that.
Kei feels like an antihero
 attempt, which is cool, but he needed more time to transition between 
these different mental states he went through, so his core personality 
stays consistent and clear to the audience. Especially since we're not 
given the details of his personality and backstory before getting thrown
 into the ajin adventure with him. 
But well, by looking at the way the events are ordered in the series, I'd say it was the intention to tell the story this way. 
..I guess the conclusion of this lengthy paragraph is that it could've been done better?
  
I also hoped Kei's childhood friend was going to be a secondary main 
character, but he was pushed out of the story rather quickly and only 
returned in season 2. He was his most loyal ally and it was a shame he 
got so little screen time. 
Kei's reason for ditching him made no sense when taking into account he 
saw no issue in staying with this old lady further on in the story. 
She's also a regular human, is she not? How was she in less danger than 
his friend? And the place he stayed at was a secluded village (where 
everyone knows each other, as well), like the village Kei's friend 
offered him to stay in, yet for some reason he didn't want to go to.  He
 just gave him a
"that won't work", and that's all the reasoning we got.
The series is clever and not difficult to understand by any means, but 
Kei cares so little about others, he doesn't wonder much about anything,
 and so his inner monologues don't help the audience understand at 
times. It's strangely fitting for his character, though. 
But well, the series keeps you hooked. Most viewers probably won't 
relate to what I've just said, because not everyone has the need to 
dissect an otherwise great anime like a maniac. 
Some matters feel rushed, but you can make the argument you're simply 
not given the time to be bored. The show keeps on going and you want to 
know what happens next. 
A definite watch. The main theme you hear throughout the series is one of the most awesome musical compositions I've heard.