This comment was meant for chapter 40, but chapter 41 was release as I was writing it, so I'll put it here instead:
As I've seen some comments here that appear to have missed some details. I'll try to resume to help in keep up with the Author's Mystery Box, and even give who I predict who's the actual killer
In regards to "Fake" Miyu, from what has been showed and mentioned, we can assess that she's not the real one, and is only impersonating the "real" Miyu, especially when on chapter 17, the start of Volume 3, the colored first page shows the two walking together, and it's not implied to be "Fake" Miyu reminiscing about the past (at least in my view)
As for the "Fake" Miyu appearance, it's mentioned that although she looks very similar to the real one, there are some differences here and there, such as the eyes, both in shape as mentioned by Hongo, but also in color, as "Fake" Miyu's eye are more pink, while the eyes of the "Real" Miyu were more orange-ish (as per the colored page of chapter 17). And also from a lack of a mole on her stomach, which is mentioned in chapter 2 that the real one had, but in chapter 34, where the "fake" Miyu fully undresses herself there's no mole on the stomach.
While her birthday, being the same as the "real" Miyu, it's said by the "fake" Miyu, it's mentioned by herself in chapter 13, that she was born in a toilet and that her mother didn't even know what date it was, so she just picked an easy to remember date, which in this case was the 3rd of March (3/3).
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Now in regards to the suspects of being the killer
I don't think it's Highway Suguru, because also in chapter 34, it's show that he was searching news about "real" Miyu's death while he was by himself in a train, and that he sounded rather surprised in finding out that she was actually dead, and that the pool of blood belonged to the "real" Miyu.
As for Yuushin, because so far, the story does follow a third view mostly centered on his perspective, there's a possibility that it could be him, but I also don't think it's him as of our current evidence, due to chapter 1 showing that Miyu had messaged him at 17:02, and he messaged her back from the hospital (Though we don't know at what time he answered) on the day of the festival, which if I had to guess, could have put his injury happening at 16:00 or a bit later.
And this is partially supported by the fact that Suguru had been following him during the day of the festival and was the one who called the ambulance.
(as for why Yuushin fell, it's said by himself that on the day of the festival, he encountered Murai, who act as if he was going to hit him, Yuushin overreacted and ended up falling the stair. Though this one is an unreliable one as well, as it's from his perspective).
Now, because of this chapter, is Hongo Haruto or his brother or both, the killer? I will be honest, I also doubt it. Why? Timing, simply put the story that Arisugawa Hina told, just don't make sense, because if she saw Haruto walking out of an alley without his cap first and then heard "Real" Miyu hitting the ground, there's simply no way that this could've happened, as Haruto would need to have super speed to somehow push Miyu out of the building and then get back to the street, as it would literally just take no more than a few seconds for Miyu to fall from 18 meters.
But also, from what we saw between the Hongo brothers and their relationship, it's really hard to say that the two would just cooperate to just kill Miyu and them go back to the older brother being an asshole to the younger one.
Of course, could the two have left the house with the same clothes on the day of the festival, and Arisugawa simply mistook the two for the other? Possibly, as we only have Haruto's side of the story of his brother's NEET life and how long he's been like that, and to a certain point, Haruto does have a reason to kill Miyu as he likes Yuushin, so he might've committed a homicide of passion, but by his reaction in the last chapter of finding out that the current Miyu is a fake one, it doesn't appear have been him, unless he's a damn good actor.
And the older brother, doesn't appear that he left the house before his cover was blow, however this is just an assumption of mine of the character, nor does it looks like he would have had any reason to want to kill Miyu or even bother himself to kill someone.
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Now who do I think is the killer
This will sound like a curveball, but it does make sense some sense, the killer of "Real" Miyu… Was "Fake" Miyu's father.
This assumption of mine is based on certain points:
1. In chapter 6, in the first page we see the scene of "Fake" Miyu looking at a burned apartment in the past, possibly her home, which on the context it was being told, it does appear to imply that it was caused by someone.
2. In chapter 12, page 20, we see what could possibly be her parents fighting, or maybe her mother making out with a lover, or something else, and she's sitting outside of what appears to be the apartment showed in chapter 6.
3. Chapter 13, "Fake" Miyu was born in a toilet, which could imply that she was born from some affair or simply that she was ignored by both of her parents.
4. Chapter 7. The letter "Fake" Miyu receive, she says it was delivered to her by mistake as the two are name "Fukagawa Miyu" and lived near each other
5. We don't know what happened to "Fake" Miyu's parents or where they live/lived
6. "Fake" Miyu looks like "Real" Miyu, but the opposite is also true, "Real" Miyu looks like "Fake" Miyu
And since we haven't fully seen or heard from "Fake" Miyu how her family was, but we've been given some details to assess that it was a rather bad family, and with most likely a lot of issues. From contextual evidence, we can assume that someone burned down the apartment she used to live in, someone knew her name and address (or general address), and this someone also knows who the "fake" Miyu really is.
Taking all of this, there's a good chance that "Fake" Miyu's father is the killer, since we never heard of her parents, he might live in the Gunma prefecture, he could have the reasons for wanting to kill her.
So what could have happened is that the sent letter was meant for the "fake" Miyu, but because the "real" Miyu also lived close by, she thought it was addressed to her instead, and "real" Miyu was going to the festival to meet with Yuushin, and because her appearance is almost exactly the same as the "fake" Miyu, the father of the fake could have mistaken her, and thus ended up killing the "real" Miyu.
What about the sports cap though? In my opinion, very likely a red herring. As a simple sports cap can be bought anywhere by anyone, so unless there's any hair of its owner still stuck on it, and they decide to make a DNA test with it to find out who owned it… I don't feel like that it'll belong to Haruto or his brother.
