Immediate reaction: This feels so dooooomed....
Comment reactions: The discussion has been stimulating. People have drawn up a number of interesting parallels and contrasts. It's inspired me to share my thought on how Yuu and Nanami compare.
Yuu is clearly naive in love and/or romantic relationships, but that makes perfect sense with her lack of experience. Some people just start late with the whole liking people thing--and it can come on very suddenly. She seems to be dealing with this (new to her) situation all right to me. The thing that really struck me was the close of chapter 17 (when the plot of the play is revealed) as we see a conclusion materializing in her head. I found that sequence very pleasing because my take away was that Yuu wasn't going to be just another dense-about-emotions character. I think Yuu's neurological wiring and/or emotional disposition is less familiar to most of us,
I am with everyone thinking Yuu is headed straight into a heart trap, but the real tragedy for me is Nanami. Nanami is self destructing and Yuu is one of the innocent by standers (the other being Sayaka) she is sort of dragging along for a bit on her way. She thinks she can or she is trying to handle more (self-imposed) responsibilities and expectations than she can. And the girl cannot let go (on her own).
I think Touko truly cares for Yuu, but her sister still seems to be too more important. She has to keep Yuu at a distance (by teasing, not talking too deeply, too long about herself, etc.). It was naive of her to act with Yuu like some robot or forever-puppy-type who would not develop feelings while being in an overtly romantic relationship. It was naive and a bit selfish, but I would also call it hopeful and desperate. If, upon realizing Yuu's feelings, Touko tries to dismiss them and disconnect, they are both going to suffer greatly; be it grief, betrayal (Yuu), hopelessness (Touko) nor withdrawal (either). I am not sold on Yuu's ever more apparent attraction being the main catalyst, but it will not mix well into whatever welling up in Touko's situation.
One last thought (re parallels): Touko is not the only one fighting for her image (or images). Yuu is trying to deny her perceived loss of her stolid persona. (She's feeling affected.) Yuu's struggle seems much more of a bittersweet (maybe more bitter than sweet) coming of age tale; whereas Touko's facades breaking down seems like it could be a more lasting scar on her life. It's really interesting how these two and their stories are interacting.
This manga is so very refreshing. I have no clue what is going to happen, but the romantic in me is looking forward to it. This has romantic tragedy written all over it!
P.S. +1 to Nakatani doing a story for adults. Or just anything post high school, really.