Haven't read all of the replies.

But I think it's someone who has died a long time ago.
I mean, someone had to have planted that scroll with the information under that house.
And I believe it was the first Mangekyou person, and it seems to me that it has been a time ago.

My guess is, that we'll never see him/her.
Or it has to be in a flashback, but I don't see how. Cause there aren't any old Uchihas left [img]i/expressions/face-icon-small-tongue.gif[/img]