Behind the mask...

[cypriottiger]

...is the will of the Juubi.
Think about it, the chakra was divided into the 9 biyuu, its body became the moon (or is gedo mazo according to other theories), but what about its consciousness? That mind of malice, destruction and hatred apparently disappeared.
I believe that the Sage could not destroy the mind of the Juubi, and so through Izanagi he embodied it as a human being.
Tobi always speaks of becoming complete, by regaining his chakra and body.
Tobi has extensive knowledge of the sage and his sons.
Tobi empathized with the jinchuuriki, as though he knew what it was like to be one, having seen from within the Sage the fear in everyones eyes of the monster within.
He could have realised that without the rinnegan he cannot seal his chakra parts together, so when madara gained the eyes, he made a deal with him. To reseal all the parts together and become complete, and in return he will lend him his chakra and allow him to complete the moon's eye plan, however i suspect he intended to betray madara. Either way it explains how he knows of madaras history with hashirama, and why he is No One.
The Sage named his biyuu, but as the Juubi he had no name.
Okay im done any thoughts?

[MagicianHisoka]

He might have left it in the body by separating it's chakra from its will and using only the chakra to create the bijuu. Or he might have split it between the bijuu too. (All speculation. Personally i hope you're wrong about tobi being the jyuubis will)

[FenixMarco]

It's gonna be someone that's gonna surprise as all (obviously), but never once mentioned in the story. Like how the 5 kages appeared.

[Airflash]

...is the will of the Juubi.
Think about it, the chakra was divided into the 9 biyuu, its body became the moon (or is gedo mazo according to other theories), but what about its consciousness? That mind of malice, destruction and hatred apparently disappeared.
I believe that the Sage could not destroy the mind of the Juubi, and so through Izanagi he embodied it as a human being.
Tobi always speaks of becoming complete, by regaining his chakra and body.
Tobi has extensive knowledge of the sage and his sons.
Tobi empathized with the jinchuuriki, as though he knew what it was like to be one, having seen from within the Sage the fear in everyones eyes of the monster within.
He could have realised that without the rinnegan he cannot seal his chakra parts together, so when madara gained the eyes, he made a deal with him. To reseal all the parts together and become complete, and in return he will lend him his chakra and allow him to complete the moon's eye plan, however i suspect he intended to betray madara. Either way it explains how he knows of madaras history with hashirama, and why he is No One.
The Sage named his biyuu, but as the Juubi he had no name.
Okay im done any thoughts?

yeah, maybe he's juubi's soul who's escape before he got sealed by SO6P... madara said that kyubi was a creature lack of intellegent.. it means that juubi was intellegent!!!