The mysterious person is Hagoromo's brother.

[Namar]

The logical explanation here, if there what you see is a person not a cave or a hole or the hill wall cracking or something
All peoples there on the Gravity field will aware of him especially Kaguya because she is facing on that person direction.
Even If Obito turn back, He will notice it.

That person that stand there is big/clearly visible to the naked eye and even us a reader able to saw him.
It unlogical that Naruto characters not aware of him.

That's mean the silouate is not a person but rather something on the wall.

And about Hamura it hard to speculate about him currently.

Thanks.

Don't think too hard Dude.

 

[Vatsa]

it's orochimaru. i'm calling it!

 

[MAN OF SIN]

Wait until the viz image comes out.

if its someone usefull, why didnt they step in and do something already!?

He probably was the one that screwed with the gravity.