Who Is Tobi?

Any acknowledgment of him being Uchiha Madara is always come from his mouth only. Even if anyone else admit his Madara identity, for example: Uchiha Itachi, it was still doubtful that they found it out themselves as there were no possible way to cross check what Tobi/Madara said about who he is (except for Tsuchikage, the only known living person who've ever fought him and thus had seen his face). Even the 4th Hokage can be manipulated since Madara lives decades before.

Despite acclaimed as the strongest Uchiha of his days, not to mention his possession of Eternal Mangekyo Sharingan, Madara as Tobi never "truly" fight or use his EMS.

No one knows whether he has sharingan on his left eye, since it was never shown.