The Masked Man is an international criminal, part of the organization known as the Akatsuki. When first introduced as Tobi he was the goofy idiot that annoyed most of the members of the Akatsuki, especially his partner, Deidara.
After Deidara's death, Tobi ditched the fool-persona and took on the identity of "Madara Uchiha." While in his "Madara Uchiha" persona, The Masked Man is a ruthless, manipulative rogue ninja who wants to destroy reality. He is calm, collected, intelligent and very mysterious.