Critiques out the way first: Ajin's beginning chapters are ass. They aren't bad by any stretch of the imagination--compared to the vast sea of manga out there, it's a pretty damn good introduction. But compared to the greatness that is the next 80% of the manga, the opening chapters are pretty fucking weak. The issue with the opening chapters is that the author has no idea what he wants to do with the characters he wrote out. They all appear one dimensional and lack any coherent sense of depth--much like your average shounen series.
Yet, as time progresses and the chapters move along, you begin to piece together the stories each character is trying to tell. Each character has a very evident sense of motivation and you know what's driving these characters to do what they're doing. They're self-aware of the actions they take may not be the most morally accurate, but its just so painfully human you can't help but empathize with them in the situation.
Unless you're Sato, Ajin's main antagonist. What makes this villain so damn good is not that he is the cleverest motherfucker on the street, no, it's also because he's a gamer. Sato is the personification of those fucked up thoughts--the crazy what-ifs you sometimes catch yourself fantasizing about. Sato's the same reason why people love the Joker so much. Sato takes those what-ifs and embodies them into reality. His violent tendencies paired with his wit makes every one of his preparations so interesting. Its this stark juxtaposition between Sato's chaotic and violent whimsicality and the organized, rational nature that is Nagai Kei that make each conflict so compelling to read.
By the end of the series, you'll see entire stories being conveyed in three textless panels. From facial expressions, to the way the panels are framed, the reader can grasp the rich history from characters they might've only spent very little time with--giving them incredible nuance despite the unfamiliarity.
Ultimately, it's Ajin's ability to convey many things in a single story that makes it such an incredible piece of work. It's never shoving things in your face but it asks the reader to really take the time to understand what's trying to be communicated across the page. Read the series or Imma have to crack out the belt
