Shinkurou's backstory was too contrived, and the amount of gloom it lumped on him made him impossible to relate to as a character. He hardly showed a single normal human emotion outside of depression throughout the entire book. The author just seemed to be obsessed with pushing how dark and dreary this world was, to the extent where he wasted pages on talking about all kinds of horrible crimes going on and dwelling on Shinkurou's crushing depression rather than talking about interesting things like why on Earth the apartment complex Shinkurou lives in is somehow completely immune to attacks by even the biggest crime families in Japan. I mean it doesn't just have plot armor, the book literally tells you the place he lives is invulnerable but then never explains how or why.
The fights are all one-sided with Shinkurou either winning or losing by a huge margin and the way the climax plays out doesn't make Shinkurou look very heroic at all.
I get that the author was going for (unrealistically) gritty realism, and that the main point of the story was how Kurenai changed Shinkurou's life, but I think the author needs to cheer up a bit and spend more time on his characters than he does on bemoaning how awful society is.