You know? I understand Yuu because I kinda have been there too. When my father died I was so shocked that I was really carefree and laughing at his funeral like I really couldn't process the fact that he was gone forverer. It was like a defense mechanism inside my mind bc I couldn't deal with all the pain at that time, it was just too much. It was like I wasn't even there, it's so hard to explain...
It's true that traumatic events can trigger unexpected reactions but 1) I don't think that's the case, I'm starting to think this manga isn't that deep 2) more than his happy attitude (which is shared also by all the others character, my head still hurst when I remember those three getting drunk two seconds after a fight), what really annoys me is how he (and the others) always forgives Guren... no, it's wrong to talk about "forgiving", you forgive if you got angry and disappointed by someone, but he (and the others) never showed any kind of criticism, doubt, disappointment toward Guren even though he has been doing questionable actions for many chapters and even killed what he said is the most important person in his life. His obsession with the concept of family is unhealthy, family doesn't mean accepting every action that the people you love do, family means also being able to stand against them and the funny thing is that surely Yuu's idea of family will be the most important theme and at the end everyone will praise him for always being so trusting of his so called family.
Yeah, but I can even accept Yuu being stupid (even though, as you said, getting angry with a person who killed isn't linked to the IQ) but all the other characters? Maybe they complain from time to time, but when Yuu says that he is family then they calm down and agree with him... This makes no sense to me and it's totally unrealistic that nobody does anything to stop Guren.
Gosh I hate so much Shinoa's plot, I hate that she could've been cool and strong character in a normal way but no, of course she's a girl and she gets powerful thanks to "love <3<3<3". This would've made more sense if she was awakening to every kind of emotions (like Violet Evergarden) but this girl is just getting horny wth??? Also the scene where her sister made her "the talk" when she was just four years old... the author really writes a female character and the only idea that comes to his mind is sex, love, desires. Nothing else.
I'm not gonna comment on how humiliating it is to read about Mika, how idiotic MC is (he is making me change my mind on other shonen manga MCs I used to dislike, they were gold when compared to him) and how hard I'd like to punch Guren. Guren may even have good intentions (lol) but why nobody questions him (beside some useless angry remarks that have no meaning for the plot)? Why is everyone so damn relaxed? Why is Yuu so happy every damn second when Guren killed the most important person for him and many other people? God the way this manga makes me mad. Mika, I'm doing this only for you.