Some spoilers ahead... It is true that the MC is quite flat, it takes around 300 chapters to fall in love with him (skipping the 158 chapters of the pirate ark). The thing is that his unemotional behavior has to do with him being some kind of monster (my guess is that he is a Kratos-like god-killing machine), it becomes part of him and something that at least I thoroughly enjoy. My point is that his flat behavior isn't something to discard him as a good character, I compare him to the before-wife John Wick as MC becomes a player to be feared and respected, whose main skills are his focus and determination.

Highly recommended for a relatively serious audience that is not into harem or immediate satisfaction. The thing that I find interesting and that I enjoy a lot is the fact that the author created an interesting character (A John Wick kind of guy) and puts him in different situations and stories, it's a great concept.

good call