Wow really enjoyed this episode and it fully explains Nanaka's actions at the beginning of the series.
What gets me now is the whole good friends issue that was brought up earlier. In my mind there was no reason for them not to tell Sana, *bleep* they should have got their parents to contact him 3 years ago. Instead they left Nanaka basically on her own, it seems that when Nanaka withdraws they leave her to it. <- that is an assumption from seeing how they dealt with her when she was angry at Sana.
The whole I couldn't do anything about it what could you do? How absolutely selfish is that? Shuu here is a clue it isn't about you. He realises how badly Nanaka was suffering (ofc not the extent of Sana now, as Sana has the secret post box), but how dare he decide to not tell Sana? Personally I think Sana has every right to be angry with Shuu, but luckily he directed his energies at being a lot more productive. So here is to Sana, lets hope he is able to help Nanaka get over her terrible trauma.
Asami does seem to be their relationship catalyst, as soon as she turns up something happens and Sana x Nanaka moves up a notch
Still plenty of mysteries to be answered. I reckon Sana was bullied to a point he slit his wrist.
*edit*
If he was bullied maybe it was because a super popular girl asked him out and he turned her down and then her fanclub starts targeting him