If you impress Yuri with your poems twice in a single run, the second time she will say that she wants to sit on the floor instead of in the chairs. When your character asks why, she becomes visibly distressed, although your character seems to take no notice, and eventually comes to a conclusion that her back pain must be due to poor posture when reading.
This is highly suspicious for multiple reasons.
Firstly, this is the only mention the game gives us of her having back pain. After this, it's never brought up again, there's nothing referencing it or an alternative reason for sitting on the floor, and it seems just a little out of place. In a game that paid so much attention to every minute detail, it's hard to see them having ANY throwaway line like this, that doesn't hint at something more, or further the plot in any tangible way. The only other similar throwaway line in this game occurs after Natsuki mentions that Monika of anyone should like squid, because it's in her name, to which Monika replies that the joke doesn't work in translation... which is not-so-subtle hint that Monika knows she's in a game that will be translated, breaking the fourth wall much as she does in later portions of the game. It's extremely unusual that ANY line in this game would be spent on something that matters so little, especially something as unusual as back pain.
Secondly, Yuri is seen throughout the game as the elegant, graceful, prim-and-proper one. It would be out of character for her posture to ever be poor enough to give her back pain, especially as back pain from poor posture only occurs as a longer term effect of chronically poor posture.
Lastly, Yuri, being the only character we've seen inflicting self-injury upon herself, is likely to actually subject herself to this potential back pain, due to the psychology behind self-injury. People who inflict Non-Suicidal Self Injury (NSSI) on themselves often do so for several reasons, one of which is that they believe they deserve the pain. Many people who suffer NSSI only cease to intentionally inflict pain on themselves, or lessen the pain they inflict upon themselves after experiencing a shift in how they see themselves and their personal self-worth. As we see that this shift has not taken place in Yuri after this, we can assume that unless there is a different reason for her to avoid this pain, under ordinary circumstances, she would normally endure this pain, as another form of NSSI. This is of course much more speculative than either of the previous to, but it's certainly not a strech.
I'm not sure if this occurs in both the first and second stages of the game, or to what extent this scenario is similar in these stages, but this is based on my observations in the second stage/play-through.
When you say that the main character of the book you read with Yuri reminds you a little bit of her, she once again becomes visibly anxious, and prompts you on why you think that. When you reply that how neither she nor the character seem particularly sure of themselves, she becomes noticeably less worried. She also mentions something that doesn't happen until much later in the book, saying something to the effect of "I don't know why I thought you meant that".
This seems to indicate that there is a much more significant connection between Yuri and the main character of the novel, one that she does not want to be associated with for one reason or another, and one that she does not want the player to find out.
SPECULATION TIME: Now, there's a few things Yuri could potentially not want the player to find out about her, which could be implied by this incident.
The first and the most obvious is of course her habit of Non-Suicidal Self Injury (NSSI). This is the most obvious conclusion to reach, and most likely the correct implication. It's also corroborated by the fact that during the second run through, after Yuri leaves to "fill the kettle". When she takes too long, and your character searches her, finding her engaging in NSSI, there's a brief exclamation, and then everything rewinds. Even though you personally know exactly what happened, your character is rewound, and Yuri comes back with the kettle. This is clearly something Yuri did not want you to see.
However, not knowing the character of the book or her story, this makes the NSSI theory seem less likely. NSSI is very rarely discussed in books, and though it is possible that the main character inflicts NSSI on herself in the novel, as the novel takes place in a camp that engages in psychological experimentation, and almost certainly takes extreme measures to prevent suicide and self harm. However, it is heavily implied that the novel involves the "the third eye" from Project Libitina. Should Yuri possess such qualities, this would likely also be something she wishes to keep hidden from your character. While this is a much more speculative possibility, it still seems to be logical.
Regarding the speculation about the back pain sequence, I believe you may be looking a bit too deeply into this. I think the game was simply trying to joke about the fact that Yuri has a rather large chest, and it’s a well-known fact that women with a large chest tend to have more back issues than those with a smaller chest. This would explain why Yuri got noticeably uncomfortable during the exchange, as having brought something like that up surely would have made it quite awkward. It also helps explain why it was never mentioned again later in the game; joking about it more than once could have been considered redundant.
EDIT: I would like to add that I did enjoy reading your speculation though, and it’s still very plausible. The fact that so many members of the community like to dig deeper into the game and try to explain hidden (and not-so-hidden) meanings of the game is one reason that makes the community so enjoyable for me, as I love reading theories and speculation.
That's something I had not considered. I'll admit, it certainly would explain the line. Although the tone of the entire interaction doesn't lend itself especially well to a joke about the magnitude of mammary glands, it would certainly fit in the context, and could easily be exactly what the game was referring to.
However, I can't shake the feeling that the line would have deeper meaning; every line I've seen in this game has a very distinct purpose, and almost every line has a double, triple, or even further meaning. From everything I've seen with this game, every interaction has a purpose, and I'd say that there are perhaps 10 lines in the entire game that don't build towards something outside of the line itself. And the fact is that this line sticks out, even from those, as out of place. In a game so well written, with such attention to detail and intention and purpose behind each line, I find it hard to believe that there isn't something further here. Maybe I'm just on a wild goose chase, but I feel that there's got to be more meaning here.
Thanks for taking the time to read my whole speculation. It's good to know that somebody else at least took the time to look into it. I was worried about the possibility that nobody would see value my attempt to find a deeper purpose to a seemingly frivolous interaction.
really like your theory on her trying to hide her NSSI. thinking about it some more, the fact that after what the protagonist witnessed in the hallway and the scene rewind, i feel like that wasn't yuri's doing, as we know that it's monika's who's tampering with the game. sure, yuri probably doesn't want the protagonist to see this, but she did not actively rewind the game.
i feel like this was just monika trying to show the player how messed up yuri is and make her look less appealing, but in order for the game to progress, she had to rewind.
as for the back pain, i think it's just from her breast size
Well, Yuris back pain thing is auctaully because her chest is so big. She would rather sit on the floor because its easier on her back because at a desk when you read your usually slouched over, and with the extra wieght on yuris front side, it takes a lot of effort to keep a good posture 27/7