How different are the manga, anime and movie versions of Ajin?
I have watched the anime until the very last episode so far, which is the 8th of the second season, while I also keep on reading the manga (it is still on going). First season of Ajin is totally faithful to the manga until the last two episodes, then it just changes a little so it is not a big deal. But this second season is, from the 2 episode and on, super different from the manga. Now, this does not mean anime is not as good as the manga, this second season is very nice indeed, but what happens is that in the manga there were some really interesting parts involving important character backstory. For example, 


   Izumi's which was very crude and harrowing but it seems like in the anime they have censored it, which really pissed me off. Then we got Kei Nagai's true feelings towards what is going on around him, which is not clearly shown in anime, and also and the most different thing: they have changed all the main plot. 


Everything that happens in the manga that should have been in this second season is not there, the fights, the crazy af plans of Satou, etc. This, however, could be explained because I believe the second season started when this amazing arc in the manga was not over yet, so I guess production decided to just walk the FMA path. But as I said, it is still worth watching it, it is good and adds new stuff:


   we can see some development in Izumi and Tosaki's relationship and we kinda get 1 minute or less of fanservice with cute Nagai blushing. 


So yeah basically, first season is completely faithful, second season is not but it still very good.