Well I'm going for Sage of Six Paths then. It can't be Oro, as Kabutoro would surely know/have detected this.

Madara's brother? Why? What's the motive? If Madara was then killed by the First AND Izuna didn't die as told, he could have stuck around to gain revenge on Konoha for killing his brother. But that doesn't answer the question as to why Tobi is a weak fighter right now.