Responses
I don't think he isn't necessarily happy being with Yui, he just went through hell when he went back to his parents house, so it would leave anyone drained emotionally! Hence why he looked so dead inside.
Well of course, but there being something genuinely wrong with him ASIDE from that, it doesn't feel completely genuine. This is my opinion anyways, so whateva~
Hours after finishing this, I'm gonna be sitting in the dark thinking about this ending and what it really means for them. Not to mention that Niichan clearly isn't happy, it's so much deeper than I, for one, initially realized. Now I'm just here...thinking about it 8ever.