As far as we know Zetsu is a Half Plant/Humanoid creature that carries Hashirama's DNA. To me he is just a tool created by Madara for the purpose of infusing Hashirama's cells, and/or for providing spare body parts that are infused with Hashirama's cells.
I expect that Zetsu's role will be to produce and Maintain an Undying Body for Madara to be Reborn Into, via Rinne Tensei. As we have seen, Madara is not too thrilled with his Edo Tensei situation, he had his own plans, which probably involve the Soulless Hashirama body in Tobi's underground HQ.
Zetsu may have been the Key to creating that Body.