So if we're talking about things have attention called to them again:
Fuu has some kind of paper written. Which is really strange to call attention to.
The shadowy figure appears to be an adult, in the similar vein as Yotsuha's mom or Fuu's dad.
If the figure approached him around the 6th grade, I don't know if that makes him a middle schooler or an elementary schooler. This might line up with the time around when Fuu-chan's dad dies and Yotsuha's mom flips on Yotsuha. But that doesn't really put blame on anyone we've seen so far.
But how does that make Uichi a killer? Are we still operating under the assumption that Yotsuha is the indirect target?
Also we got two new names: Yotsuha's family name (or at least the one she used before re-adoption), and Fuu's family name. I'm not convinced the name on the heel isn't the name of Yotsuha's dad. I don't think she's some secret lovechild either.
Something smacks odd about Fuu wanting to keep that paper hidden. I wonder: did Fuu figure out the time travel before Yotsuha ever got it? If you were super smart, wouldn't you try to go back in time to stop your dad from killing himself? What would you do if you butterfly affected yourself, and Uichi got to the clover before you could get it? Yotsuha seems like she can only go back so far. Or more specifically, she can only go back to 20:15 of the previous day. Which means as far back as she had the watch + one extra day BEFORE getting the watch. I think this works in some capacity since Clover wants their watch back, Uichi being the clover picker, and Yotsuha being the one who ended up carrying it away.
That still doesn't make any sense since clover is depicted as an adult, and Fuu is pretty darn small. But she is one of those people Rei seems to idolize. So maybe? What if it's the Toshiya mentioned in chapter 70? Does Fuu have siblings? Feels really late in the game to make up a new character, and also kind of an asspull since there doesn't seem to be a huge setup to it.
EDIT: I just noticed in the page where he rips the letter he uses the world "he" and that he also seems to KNOW who that person is. Is that intentional or a translation hiccup since everything else is mystified? Either way, it makes me think it isn't Fuu. But maybe it's Fuu's dad? Or Yotsuha's brother (which still doesn't make much sense)?