SPOILER
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I already suspected some kind of time loop (reason I mentioned Ichinose Family Deadly Sins in a previous comment and the "Sisyphean" in the name kinda gives that vibe), but I didn’t expect Jack to loop with us.
I was glad that Raymond eventually understood how killing Jack would break the "Pairs" motive, it was my first thought during that scene.
And going for that those who survive the loop retain the memories, it kinda explains Lucks and Zacks connection. But if thats the case, I wanna know if previous loops would still take place or get completely rewritten (Assuming Luck survived with Zack once, would that loop still take place? Like, for Luck he just looped and doesnt remember Raymonds loop).
The only thing that kinda makes sense to me is that the killer must also loop, even after dying, which would explain a lot (information advantage and killing Zack First cause he was unstable and maybe connected some dots), and it would make the mystery harder to solve.
And my guess is that the loops only end if the lullaby happens, since thats looks like the idea the killer wants.
But it's hard to theorize in a mystery with supernatural elements.
Still, that doesnt stop me lol.
Its pretty good so far. Luck is by far the most intriguing character for me, just love the way he rationalizes everything.
Taking random guesses about the possible culprit:
Raymond is out cause i wanna follow Knox (I know that u already said you wouldnt, but at least that i think it takes some merit, since u keep refering him as the detective).
Jack is out of the way, since if my guess that the culprit wants the lullaby to take place, he would just kill Raymond in the last day and that would be it.
Lotte using the plural for herself, wich is strange, so it could be some double personality? Would be kinda stupid, but it tracks.
I Agree with Raymond, Luck was so suspicious that being him its kinda impossible to believe, the same goes for Zack.
Layra, Jason, Arthur and Emily are too genuine, they would need to be "master manipulators" (cringe title, but didnt think in anything else) for pulling so much off, so its hard to doubt them.
Lewis gave a lot of info and i kinda doubt the culprit would write a diary, so it would be to hard to be him.
So that leaves Alice, Joshua and Lotte... Yeah, no idea how that would work.
And i still have no idea how to connect any possible theory with the Code and the book, the only thing that i can guess its that this is some kind of limbo, but nothing concrete...
Looking forward to future updates!