@GreenMoriyama
Thanks for taking the time to read and reply. I admit my theory is pretty skew towards Miku though I hope I did try to rationalise my reason so that it seems possible. Your point about Rena wanting to become better… I assume you’re talking about younger Rena, right? In that case, then if younger “Rena” tried her best in academic but her sister achieved similar or even better result without much effort compared to hers, what do you think it’ll do to her confidence? She’ll start believing less and less in herself, eventually becoming what she is at the start of the series.
Most of her total scores can be attributed to her love for Japanese generals, but that wasn’t always the case was it? Before she discover her love for history, what was her score compare to her sister? We don’t know as it was never shown. But remember the reason why she loves Japanese generals? It’s because of their ambitions. Ambition that she never did gets to fulfil.
Also, I forgot to mention in my theory post, but “Rena” is separate from the “Itsuki” that kiss Fuutarou, until proven otherwise I’ll treat them as separate.
Anyway, GreenMoriyama. I don’t know where you post your original theory on Yotsuba is at, so I’ll have work off from your reply and the comment you post in ch.69. And just to let you know, I have nothing against Yotsuba, but the timeframe just isn't right for me. But just to be sure, I went back and look at Yotsuba’s movement in that arc to see what sort of timeframe is needed or required.
The day Rena appear before Fuutarou is on a Wednesday, after school. A quick internet search says the Japanese school period end around 3pm. And we already know what time they need to be at school, 8:30am taken from when they overslept for the first exam. So, 5.3hr of school time.
The two instances that we have a clear indication of time is 9:40pm on Yotsuba iPhone and 9:37pm on Ichika’s iPhone. Both times are after they are done studying, and for Yotsuba is the time she prepares for sleep.
The next question is when does Yotsuba finish her training with the T&F club? From the various background in ch.44, we could see that the residential buildings and streets lamp light are on, and by the end of the chapter two members of the T&F club are seen leaving together with their bags, so about 6-7pm is when the T&F members ends their training and go home. What we get from this is, 5.3 of school lesson, around 3-4hr of training with the T&F members, and another 3-4hr of study and working on Fuutarou's problem sheets at home. A total of at least 12hr, not including morning necessity like, washing up and breakfast.
In ch.41, Fuutarou confront Yotsuba afterschool about her involvement with the T&F club on Monday and at this moment, Fuutarou’s depression has not become apparent or worrying enough that it warrants the appearance of Rena, that is until his second dealing with Nino who dealt another blow by claiming that she doesn’t care about passing, combine that with Yotsuba now. The page after Nino said that is a montage but the first two panel is from the same day, Monday. Why? Because of Raiha hugging Itsuki, Itsuki must have ask either Ichika or Miku to get her wallet for her and with that in hand she decided not to impose on the Uesugi any longer until Raiha happen and she stays a bit longer. The remining panel is for Tuesday, with the sequence being, breaktime cafeteria-Nino, afterschool-Yotsuba and hotel-Nino before transitioning to Wednesday, afterschool.
If Rena is Yotsuba, then this mean that Yotsuba literally have one day to prepare herself. Also, don’t forget who Yotsuba is dealing with here. That captain seems very pushy when it comes to training. And given how important this event is to them, along with the upcoming exam, Yotsuba won’t be taking any leisure time like a boat ride, she will either be training or studying throughout this arc, as to not drag the T&F members down or Fuutarou down.
Also take into consideration of the time it might take to find Fuutarou, even if she knows where to look for, it’ll still take some time to find and reach Fuutarou from wherever she is currently.
Let's bring up Rena choice of clothes. From what Rena said, she spent a considerable amount of time trying to look like how she was five years ago. Now, were the clothes recently brought or was it something she already own, because the only quint I could see that could own such an outfit is Ichika. If it's the latter, then Yotsuba would have to find them in pile of other clothing scatter about in Ichika’s room, compare which dress matches the most to five years ago, and she might even have to ask for permission too, all of which takes time. If it's the former, then she would have to do it in-between the time she is done training with the T&F club and studying at home. Either option is possible but still time intensive.
Then, we have the boat ride. A user on Twitter by the name of hanaimode, manage to find out where the place is base on, Nakanoike Park. (The real-life version doesn’t have the boats by the way)
While I’m unsure of the exact length of the boat they're riding but lifting the average speed for a 4.3-meter boat on the Wikipedia page on “Rowing”. As well as opening Google Maps and playing around with the measuring tool at various points of the pond that has the stair symbol. We can estimate the possible duration for the boat ride, with the shortest being 5-7 min and the longest being 25-34 min, with the average time being 14-19 min, taken from eight possible route. A very IMPORTANT note is that this, IS NOT ACCURATE AT ALL. Since the boat that they’re riding on seem to be smaller than 4.3-meter, the calculation also did not take into account the combine weight of Fuutarou and Rena, the type of boat they’re in, Fuutarou’s physical capability, and the pace of the boat when Fuutarou began talking to Rena, so do not take this as fact. But what doing all this does, is give us a very rough, if very inaccurate estimate of what the suppose timeframe for the boat ride could be like. Can Yotsuba really afford to spend these sorts of time?
Another thing to note is when Fuutarou and Yotsuba meet after Rena disappear. Fuutarou was just stepping out of that pier, supported by the fact that those beams behind him on page 16, don't have any railings on them. Even if he is depressed, it shouldn't have taken him more than 1-3 min to get out of the water and cross that pier. Also, at that moment, Yotsuba request if she can take a break from running, suggesting to me that she been running with them or her for awhile now, if she had already taken a break earlier, she would not have asked for it now.
My next question is, would Yotsuba disguise herself as Rena? Not why, but would? Up till now, not once has she ever deliberately disguise herself to fool Fuutarou. The disguise in “The Legend That Binds” are her following her sister’s plan, while “Scrambled Eggs” disguise was a requirement.
I hope those thoughts and reason explain why I didn’t think Yotsuba could be Rena.
@Lex79
Thanks for taking time to read it. If you manage to put together a theory on how Itsuki is Rena I would love to hear it.
Also, I have two additional points for Miku being Rena. The first is the hat, and the reason is very simple. While it’s used to hide her face from being recognise as one of the quints, there's another propose as well, to keep Fuutarou at a distance. Because of how wide the rim are, it serves to prevent him from getting a closer look at her face. What brought this about could be her prior experience when she was disguise as Ichika, when meada almost saw through her disguise after he got a closer look at her face. So, the hat is just a precaution if Fuutarou ever did the same thing as Meada.
The other point is the words Rena used before she says “necessary”. “Face them earnestly” now only two other people I remember have said something similar to those line, the first being Miku and the other being their grandpa. I know this is a very lofty evidence but given how likely someone could have overheard Fuutarou and Itsuki conversation, they would know that the word “necessary” means a lot to him, thus may utilize it as a way to cheer him up. So other than “Face them” the only other word that may point to her identity is “Acknowledge”.