So I just finished reading Ajin: Demi-Human and I was really impressed with how good it was. The story-telling was really tight, and the logic behind the immortals was really well thought out and I liked the author didn't feel a need to explain everything perfectly. Sato and Kei were great foils to each other, both having sociopathic elements to them, though on different sides of the scale.
Sato makes a great Joker-like antagonist who just views his life and the lives of others as a living game. His motivation is that he has none, he just wants to make a story-line for the game he is playing. Its great because he's so unpredictable.
I also liked how it ended without a gigantic fucking kaiju monster that some mangaka think they need to end their manga with. It was also kind of funny to me how Japan were portrayed as treating their demi-humans worse than the Americans, who just kept tabs on them, basically for their own protection as they'd be really useful for all kinds of heinous shit.