Well who do we think he is?
In my opinion, i know it is strange but i reakon it's Obito. Yep Obito, Uchiha Obito.
I know how it ended, he gave his eye to Kakashi and he got crushed by rocks and such.
BUT i think seeing as the Kakashi Gaiden arc was during war, and at the end when Kakashi, Rin and Yondaime went back to the area where Obito was. The entire rock area had collapsed and washed away on the river. So i think Obito was washed down stream and found by some Hidden Rock Ninja's. And they saw that he was apart of the Hidden Leaf. So they cured him with a Medical Ninjutsu user and interrogated him. Then he escaped some how and the rest is a blur. And remember Itachi said if he includes Sasuke that will make 3 people that can use the Mangekyou Sharingan. So meaning someone else has got it already. It could be Kakashi, seeing as he has the ability to open up the Mangekyou Sharingan but i doubt it. And Rin was Obito's best friend and even lover so if Obito killed her he would gain it.
I'm a big picture guy, not a detail guy.
We can see that the Akatsuki leader kind of looks like he has Sharingan eyes, if you compare his eyes to Itachi's eyes when their in their lair, they look similar. But Obito lost one eye so that is sort of awkward.
It could be Yondaime, but we know he died at the start but was it all a fool's joke!?
It could be Shino's dad! Same hair and he's a mysterious guy....doubt it big time mind you
What a surprise if it was.
What about Mizuki?? He had 1 chapter of fame and got locked away in prison. Maybe he was immensly training and was the leader all this time!
Who knows!
But what do you think?