So is Yano to blame? Nana-chan? Yuri? Nana-san? Takeuchi? There's a lively discussion on AnimeSuki spreading blame for Nana and Yano's problems far and wide. In my opinion, to quote Bob Dylan: "It's life and life only." No-one is to blame, even though everyone could have made better choices. It's tragedy: people's natures lead them astray.
Nana-chan should not have pocketed that photo, and when she did, she should have found a way to get it back, even if she didn't admit taking it. And she shouldn't have left it in a book where someone could see it. But the taking was the impulse of a girl in love. And the not returning was lack of guile. And she shouldn't have eavesdropped on Yano and Yuri. But how could she resist, when their relationship had been worrying her for so long?
Yano should not have ripped the photo up -- and he should not have slept with Yuri two years before, in the shadow of her sister's altar. And he should not have lied to Nana about it, even though, as he told Takeuchi, he just didn't want to see her cry. He was too embarrassed. And when he did tell her, he should have been more serious and contrite. But he's 16, and a macho guy. It was beyond him.
Yuri should not have seduced him two years before (yes, it was she who led). And she should not have pined away for these two years. But she is in love, too. And she envied and despised her sister. Like Nana-chan, it was emotion over reason. Who among us hasn't been there?
And Nana-san should not have got into the car. But she had argued with Yano, and he had not called her on her birthday. And she really was still entranced with the brute. She had been seeing him all summer. It was, unfortunately, her nature. It happens.
Takeuchi? I don't see any blame there yet. He has been supporting Yano all the way through, even though he likes Nana-chan himself. His advice to Yano to come clean was good advice. He just didn't take into account that Yano could not do it in a reasonable way. Now Yano may blame Takeuchi for making him make Nana-chan cry.
Nana-chan overhears Yuri telling Yano about the day her sister died, and mentioning "that day" two years before. So she knows something went on between them. After Yuri leaves, she leaves, too. Yano tries to bring her back, saying he loves her, but he also blames her for not being more forgiving.
Takeuchi sees the quarrel, and later advises Yano to explain things to Nana. Yano says he can't because he doesn't want to see her cry. And anyway, he doesn't feel guilty that he did what he did with Yuri, just that he made a mistake.
The next day at school, Yano brings Nana-chan the textbook she left behind at his place. She tells him she loves him and asks him to tell her the truth. He makes a joke of it, but eventually does. She told him she could take anything he told her, but she is inconsolable. For the first time, Yano begins to regret what he did. In the preview to episode 12, Nana says she can't go on with Yano.