The more i've thought about this, the more I believe that when Tobi say's he is "nobody" he is actually speaking the truth. Here are some different things we know about Zetsu, and why I believe that Zetsu is far more important than we give him credit for.

First of all, I believe Zetsu and Tobi are the same person, and I believe Tobi is a unique and special Zetsu clone made from contact with the DNA of Madara Uchiha, and perhaps Hashirama Senju.

How do we made those conclusions? well we can assume that Zetsu himself was a construct of Hashirama Senju based on his unique ability similar to the wood style itself. We also know that Zetsu can make clones that are very similar to those who he encounters, that can take on the appearance and even the very same presence as those! we know that based on the originally faked death of Kisame!

Next we can argue then for the purpose of the "war" and the use of the white Zetsu army. To carry out this plan in secret would have been easier, and probably more able to be done if Tobi and Madara were the same. The fact that the war was planned and the use of all these white Zetsu clones were included makes more sense if Tobi is actually a unique and special Zetsu. We know that the Zetsu clones who are attacking will attach themselves to the shinobi they attack and suck out their chakra. To that end, to have a war where there are many Shinobi present, and the Zetsu clones (while not that hard to defeat in standard combat, but are very numerous) can latch onto these shinobi and suck that chakra out, giving it to Tobi, who can then use that massive chakra to control the tailed beast and actually use the Rinnegan/Sharingan.

Well, that's part of my theory! hope you enjoyed the read (sorry if someone said something like this already, I didn't see it if they did).

Or maybe Tobi is special zetsu who in the past copied Madara Uchihas Abilities and Ideals and Thats how he got Senju DNA and Sharingan O.o Lol Jk