Part 4: Fan Reasoning
Of course, we still don't know why Yuu "disappeared" after Nene got killed. Is it just the Oumahito screwing with us? Or is there going to be a twist where it turns out Yuu doesn't actually exist, and is only a delusion created by Nene? That might explain why he didn't show up in the first three chapters.
I hope that this is an option Sigma's avenue of thinking can actually go.
Obviously, Yuu is just a nervous delusion on Nene's part.
The Oumabito's time distortion has altered the timeline so that Nene's mind has become unstable. Yuu didn't appear in the first three cases because Nene hadn't gone insane longing for companionship.
Wait, Sigma, I have a question.
If Yuu is a hallucination, then how did Rin see him in the last case? Nene wasn't anywhere in the garden when Rin discovered Yuu's corpse.
The Oumabito must've altered the timeline so that Rin started hallucinating too.
I... I guess?
Q.E.D. Begin tuning.