I agree with the guy who said it might be Obito...I mean I do have doubts, but we only ASSUME he died beneath all the rock. My biggest doubt that it's Obito is actually because of the man who attacked Minato was a full adult. Obito would only be a teenager, and the man was the same size as Minato. Now, if Zetsu had a small part in it, it makes it a little believable. But still, there's room for doubt.
It has to be someone Madara knew personally. He seems to know who is behind it all, only by a second he came back, he knew. BIGGER question: How is Kabuto's someone elses assistant? Exactly how old is he?