The spy being......Kakashi
Now, I'm not saying for sure that Kakashi is/was the spy, but at this point I wouldnt put anything pass Kish, especially if it makes the story that more interesting.
If Kish somehow made Kakashi the ultimate traitor that he portrayed Itachi to once be would be pure EPIC. Kakashi reasoning the death of his father and having no one there are for him. He's shown no real emotion, so he would be the best person for it, fooling eveyone.
So how would you feel about Kakashi being the true spy of the series???
Do you think it's possible??