Tobi

Nothing much is known about him jet. The Kyuubi mentioned him. He said that Uchia Madara has chakra more sinister than is own.

Speculations
Some people believe that Uchia Madara is the third Uchia clan member who has the ability processes the Mangekyo Sharingan. Some people also think that Uchia Madara is the person on the other side of the valley of the end.