Tobi

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

Speculations
Some people believe that Uchiha Madara is the third Uchiha clan member who has the ability processes the Mangekyo Sharingan. Some people also think that Uchiha Madara is the person on the other side of The valley of the end. It is later learned that Tobi of the Akatsuki is Madara and the Mastermind of the Akatsuki.