Okay, there were a lot of weird things going on in this chapter. To summarize everyone else's observations, as well as my own theories:
This nameless child who looks and acts exactly like Tsukasa is claiming that this house, the Red House, is actually his home. Furthermore, he seems to be a living person, but he obviously knows something. It's unlikely that he's a descendant of the Yugis because Amane and Tsukasa were both around 13 when they died, and if the Red House really was the Yugi house, then according the newspaper article we saw last chapter, there were only four people in the family: the two twins and, presumably, both of their parents. No other kids to carry on the family line.
Also, he simply knows too much to just be an unknowing descendant of the Yugis. For one, that creepy song he was singing while waiting for Kou to wake up. He specifically mentioned the moon, which is one of Hanako's big symbols and callbacks. He calls Nene by her name, despite the fact that she never properly introduced herself, and Kou only ever calls her Senpai, so he should have no way of knowing it. Then there's the fact that he's even aware of the existence of this mysterious "they" he mentions, which a normal child shouldn't know about unless he was close to the Far Shore. Why do they not want him to leave the house? Are they protecting him, or are they protecting others from him?
If all the spirits of the people who died in this house remain here like the child claims, does that mean that parts of Amane and Tsukasa's souls are somehow stuck here? Maybe this child is like, a fragment of Tsukasa's soul splintered off in the Severance, and it remained here because its ties to the house it died in were sustained (just like how the mermaid's curse on Yashiro, and therefore her bond to Hanako, wasn't broken by the Severance either). Though that doesn't explain how the kid would have a pulse, it kind of makes sense otherwise, maybe?? And does that mean that the one that Nene and Kou heard this child talking to (the one who was crying over his broken toy) really was a version of Amane? Where did that kid go, anyway? He wasn't there when the door to the room opened.
Plus- this is pretty small, but the way he kept following after Kou and bumping into things to get his attention seemed kinda suspicious to me. He was never shown to be clumsy at all until that point. It felt like he was trying to evoke Kou's pity so that he'd be allowed to follow him...
Who exactly is this kid?
And something else I noticed from a previous chapter:
I'm not sure if it'll show up, but this picture is one of the extra pages from chapter 20, showing some of the main characters entering Kamome Academy's student orientation with their families. We see Nene with her parents, Mitsuba with his mom, and Kou with Teru and his baby sister Tiara- but the Yugi twins are alone. And if you look closer at their picture, you see that the twin whose face is visible actually looks to be
Tsukasa. This twin's pupils are in the center of his eyes while Amane's/Hanako's are always drawn to be a bit wider and connected to the top of his iris, if that makes sense. But why would
Hanako's face be scratched out? In the other pictures, scratched out faces seem to mean that the person just isn't relevant, like Nene's parents or Mitsuba's mom. But Amane and Tsukasa are both important characters; Amane is arguably MORE important of a character than Tsukasa. So why is either of their faces scratched out in the first place? By chapter 20 we already know that Tsukasa is Amane's brother and the one that Amane killed, so there would be no reason to hide either of their identities. It just strikes me as odd.
Also, going back to the Red House: we know that it became famous because the death of a four-person family living there, suspected of being a family suicide. This could presumably be Amane and Tsukasa, along with their mother and father. So if the Red House really is the Yugi house, and the Yugi family is the one that died, they should have died at around the same time. So where are the Yugi twins' parents in this picture? They should have been alive at this point. So why are the twins alone? Why doesn't it seem like the parents ever took action even when it was clear their son (Amane) was being abused by someone? Were they the ones doing the abusing, or were they just neglectful enough not to care? Or maybe Tsukasa was the real villain here, and he threatened/hurt their parents into silence? It's understandable that Adairo wouldn't want us to see Amane and Tsukasa's parents at only chapter 20, so that could explain why they weren't illustrated in the picture. But whoever the Yugi parents were, I don't think they were very good, well... parents.
Lastly, let's not forget that Nene still has the key w/ the rocketship charm that young Amane had dropped in chapter 22. That totally unlocks something in the Yugi house, I bet.
Thank you for the high-quality translation!