Im pretty sure its Izuna... Because I did a bit of reasearch and found an old picture of Tobi with his older mask. when I compared it the hairstyle to both Madara and Izuna it turned out closest to Izuna....
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Also see: Naruto Shippuden Episode 136
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By looking at these pictures I conclude that it is Izuna Uchiha - Madara's Brother.... If you looks closely enough u can see the similarities. The only difference now is that Izuna doesnt have his hair tied back....
Also I would like to point out that both Madara and Izuna are twins by the look of things.... Madara just has his hair let out loose and his brother has it tied up...
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What do you think?? Who is behind the mask???