Spoilers ahead
But after the chapter I mentioned, he isn't any more the protagonist. Kuroda, that already had a gigantic amount of screen time, gets even more attention, the main protagonist is left behind and Kuroda is basically everywhere.
Kitano just becomes a passerby in his own story.
Ryoko, from a strong willed girl, becomes a shouting 24/7 machine.
Takehisa is good and his changes is believable he but has even less amount of screen time than Kitano's dad cat or Ikuko. And we never see the gradual change. He just changes.
Kitano's mom sad story that nobody asked for. She was assaulted by her cousin and she was ok with that.
The whole plot line of the red hair guy was left behind.
The whole plot line of Ryoko's dad was left behind.
Nobody is afraid of Kitano even when they don't know the truth. Kuroda was shouting at him and so were many other characters. And nothing happened.
The photographer girl was going on toe with kitano's speed. Like what?
A ghost was literally introduced in the story.
Ikuno promised to a nobody (a complete stranger) to beat kitano when they were already friend and she literally did it. And everyone, even Ryoko, was ok with that after finding it out.
The blonde boy that could see her pain while she was beating the crap out of Kitano was also what the hell? moment. The author tried to be deep but failed to do because the dialogues are poorly written.
The fact that she was on pair with Kitano's speed while she herself admitted to be weaker than her father who was beaten by Kitano is another what the hell? moment.
After chapter 40, only females character are on the main spot. And this continues until the very end. Everyone forgets who Kitano is in the meantime. And from this point, we spend like 99% of the time following the story from another characters pov.
The whole mockery of the foreigner that couldn't speak japanese was also unnecessary and irritating.
The manga goes from 10 at the start to a very generous 6 at the last chapter.
