Intended to throw some coins for a while now, but was too lazy to login, anyway:
1- I don't think the "unassuming middle-aged guy" is the Grim Reaper. So far, entities in this manga have been very japanese, and "Shinigamis" (as would be the equivalent) are not only very vague in old literature, but also very diverse in modern popular culture, so would be pretty much impossible to predict (and as for the Grim Reaper, most stories will depict it as an entity that takes people "when their time comes" and
I find it very hard to believe that it would attack Romm just because he saw it).
2- I won't entirely deny that Michiru's caterpillar may have some relation, but let's not forget that caterpillars don't stay in their larval stage forever and You (Michiru's sister) already said she replaces the caterpillar whenever they change to cocoon/chrysalis stage, so
the current "Pii", is not the same one as the "Pii" from a few weeks ago, so again, I doubt a pet that isn't even around anymore is Michiru's "forest guardian".
3- The voice mimicking ghost, Miko have been seeing random ghosts for a while now, and you people never assumed they would be more important that they actually were, so why is this one different?
"
Because her father warned her to not open the door"?
It was
dangerous, of course he would warn her;
"
Because it mimicked You's voice"?
Considering what we have seen so far, mimicking voices is hadly impressive, some people can do it fairly well, and some birds amazingly well;
So as far as I'm concerned,
that ghost was just a random that will probably never show up again, and yet people are letting it derail their judgement.
And finally,
4- Going back to the "unassuming middle-aged guy", back in the holpital, Romm explained to Miko how perspective is important and any given thing can be different for different people, and the difference between him and Miko was basically a difference in opacity,
Romm can see ghosts, but still see what's behind them,
while Miko sees them as completely solid, unable to see what's behind. Of course, this case doesn't seem to be the same thing, but it's also possible that this is Romm's years of experience comming into play, so he can see what the entity truly is (or as others pointed out, it could just be Miko's seeing it for what it really is, an old man), many possibilities, that we may find out in a month or two, but hopefully not three.
That's it, sorry for the [
Wall of Text].
Also,
Wrong manga.
