Ajin is like that one 29-year old cousin who still lives in his mom's basement, but when you actually try to interact with him, he's pretty cool.
I have to start this review by saying that I am not a fan of the anime adaptation of Ajin, the anime was animated poorly, and diverged from the source material, which we all know from experience, is not good.
Let's start with the characters. The mc and the antagonist are really the 2 main characters who stand out as great in this series. The dynamic and relationship these 2 have make every single chapter exciting, suspenseful, and entertaining. I especially took a liking to the mc, due to how relatable I found him to be. He grew up with a extremely mature mindset, and took on many responsibilities that even adults wouldn't want, as a child. He was the perfect student, modest, quiet, smart and outstanding. The antagonist however, was quite the opposite. He was psychopathic, cold, absurd, and insane. As these 2 battle it out, exchanging blow for blow, the world shakes, and their ideals clash, resulting in great character development. The side characters aren't bad either. All of them were unique, and felt important enough to the story, so that we would actually care about them. My favorite side character was okabe's edgy stoner twin cousin, who was badass, hilarious, and smart as hell.
Next is the story. This is of course a shonen series, so don't expect much of fantastic storytelling or the use of literary devices. However, the pacing is great, and the arcs don't feel repetitive at all. So unlike another anime that uses the similar power of coming back to life *ahem* I'm looking at you, re:zero, the story is constantly moving forward, and the events feel fresh in your mind.
Finally the art. The art is great in my opinion. You can see clearly the evolution of the artist's style throughout the story, as it changes from a more cartoony, stereotypical character designs to a semi-realistic art style that gives the manga it's own separate identity. The 2 paged panels adds to giving the story a dramatic effect, which adds to the enjoyment of the reader.
Overall, the ajin manga is truly a hidden gem of the animanga community. It's well-paced, enjoyable, and relatively original, despite the main power system being used in other animanga. If you like fast-paced action or modern-warfare mangas, this is definitely the one for you.