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1)It is revealed that Tobi must be of old age which is why it can't be Obito or anyone around that generation

2)If Izuna Uchiha was second to Madara why hasn't Kabuto used Edo Tensei to bring him back...because he can't since Izuna(Tobi) is not dead.

3)Right before Madara dies he unlocks the Rinnegan. Have you ever wondered what happened to Madara's eyes when he died? They were implanted to Nagato and so forth. Before Tobi fights with Konan he clearly states to Konan that the Rinnegan is not Nagato's it's he's(Tobi). When he said that I just knew who it was. Think about it the Rinnegan Nagato had was actually Madara's eyes. Tobi says its his eyes because Madara took his younger brother's sharingan and whose his younger brother IZUNA UCHIHA.

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it has to be obito, i think in one of the mangas it shows tobi with his mask off. the right side of his face is old looking and his eye perfectly fine...probably where the boulders crushed him. the other side is smooth and young looking with the eye allmost white with nothing there. where he gave kakashi the sharingan. Which is why i say Obito...is tobi...tobi is obito
http://www.konoha-village.com/wp-content/uploads/2012/08/obito-tobi.jpg

go there...or just google tobi now...ITS EVERYWHERE