I just decided to reread the ones here on Dynasty and I have to agree that the "I just did what anyone would have done" line stands out quite strongly.
It is interesting because, from very early on in the story, it appears that Yuu thinks of Nanami-sempai in a special way. In the first chapter, when she is seeking advice about the love confession, she decides to ask her because of the fact that she sees a similarity between them based on Nanami's statement that she has never gotten exited about being confessed to by someone.
Then, as the story continues, she repeatedly notices Nanami's true feelings that she is able to hide behind a mask of competence and bravado from even her closest friends. She simply understands her better than anyone else. I don't see any indication that she has this kind of insight into other people's thoughts or feelings.
She also tends to bend over backward to help Nanami and protect her from, among other things, herself. She is baffled by why a person who is as fragile as she knows Nanami is would put herself in positions time and again where she has to perform in the public eye, with the price of failure being the dissolution of her carefully designed persona that she has built atop the frightened, shy little girl who couldn't make friends and hid behind others. Yuu, who is the only person who knows the real Nanami-sempai, seems dedicated to protecting the public face that Nanami has built. When Nanami, and now Maki, notice how special Nanami seems to be to her, from other people's perspectives, she consistently responds with a variation on the theme of "That is what anyone would do." She doesn't recognize her special treatment of Nanami as being special. (I wonder if her opposition to the play isn't at least partially motivated by the fact that she knows it will be nerve wracking for Nanami?)
I really would like someone to ask her, "If that is what anyone would do, is it what you would do for anyone? If it were someone other than Nanami-sempai, would you go so far to support/protect them?" I really would like to know the answer to that myself! I don't know if she would say yes or no... (My gut says that the real answer is no, that she wouldn't go that far to protect someone else.)
In a lot of ways, it seems like, if you were not privy to Yuu's inner thoughts, you would think that she was in love with her fairly early on. Her behavior patterns certainly make it seem that way. In fact, you might think that she was in love, but either too shy or unsure of herself to show it openly.
In any event, the line at the end of chapter 5 speaks volumes: "I couldn't just simply give up. But I didn't realize it at that time."
I really do love this story! It is very well written.