Buff, you forget that Kouko is a complete idiot. She's spent years making sure that Mistuo believes she is this perfect woman, a perfect match for him. She hides all of her flaws with a psychotic diligence. Her week-long preparation and retrieval of her scrap books (6 volumes of only her an Mitsuo!!?!) shows how delusional she is. She took a single phrase that Mitsuo never took seriously even though he remembers it too (how could he not with her around), and turned it into her life's goal from kindergarten until now.

Banri is the only person who has seen the real Kouko in probably a decade, other than maybe her immediate family (but we have no idea what they're like). He likes the real Kouko, not this illusion she's created. That's why his confession wasn't just thrown aside as pity. The two of them understand each other on a level few others can. Banri is an illusion to his family and former friends. But the reason he distanced himself from them is because if he pretended, he would be a fraud to himself. Kouko's persona is a complete fabrication. Her confidence, assertiveness, bitchiness...all fake. What she is is an idiot, desperately and hopelessly in love, and an extremely diligent worker.

I'm also really glad that they cast Horie Yui. I can't imagine anyone else playing her. Kouko has so many similarities in personality to late-volumes Naru it is uncanny, though in personality she's more like one of the two female leads in Haganai.

Regarding Chinami, I agree with one of shinta's earlier posts. If she had outright cried, I would have been convinced she was faking it. There is something...off about the way she reacted to Kouko's panic-induced bitchiness. She takes it quite casually until Kouko steps on her foot. She's a year older than the other three, and in the film club, so being good at acting just like fake-Kouko isn't beyond her reach. I think she was just taken aback by the extreme level of assertiveness that Kouko went to. Something like the intensity of Kouko's attack broke the perky mask she likes to wear. The same way that Mitsuo's sudden anger broke Kouko's in front of Banri.