Tobi

also known as in Akatsuki, was formerly Zetsu's subordinate. Madara became a full Akatsuki member after Sasori's death and afterwards wore the same ring as Sasori having recovered it after his death and taking his place as Deidara's partner. After Deidara's death, he revealed his real name and that he had been the one giving Pein orders, making him the true leader of Akatsuki.

Personalities
It is impossible for Tobi to be Madara Uchiha. 1. Madara Uchiha is as old as the First. 2. Tobi's hair isnt like Madara Uchiha, and by now Madara's hair would be white. 3. Tobi doesnt sound as old as Madara would be. He sounds about 20.

Abilities
Tobi's area of expertise remains unclear, as he has never been shown fighting or utilizing jutsu. He and Deidara were able to defeat the Three Tails, but Tobi was never shown doing anything but running from it. After the battle, which goes largely unwitnessed, Tobi claims to have knocked it out with a single attack. Deidara disagrees with the assessment, claiming that it was his "artistic contribution" (exploding clay) that allowed them to win. During his fight with Sasuke, Tobi seems to show the ability to recover from what are presumably debilitating or fatal blows. After being slashed across the midsection by Sasuke's sword, Tobi collapses, only to stand up again a few seconds later and complain about the speed of the attack, and later when Naruto hits him in the back with a Rasengan. During that fight, Tobi exhibits skills of movement underground, as he plants Deidara's clay minefield. He is also able to escape the final explosion created by Deidara. After he escaped he revealed he had the Sharingan, one visible through the opening of his mask. The demon fox mentioned that Madara Uchiha has more sinister chakra than his own. According to Jiraiya, Madara was the only person capable of summoning the demon fox, which was said to only come where human malice collects and festers. Jiraiya suggests that someone must have summoned the demon fox to attack Konoha and the only person who could pull something like that off was Madara.

Part In Story
Tobi first appears with Zetsu to retrieve the rings of Deidara and Sasori after their defeats. After finding Sasori's ring, Tobi asks Zetsu if he could join, who claims it isn't as easy as that, although he joins Akatsuki anyway. After Deidara was revealed alive, Deidara was quickly annoyed by Tobi, and choked him with his feet. Tobi later appears with Deidara and defeats Three Tails. During the battle, Tobi is seen doing nothing but running, but claims to have been the one to defeat Three Tails, though Deidara disagrees. During Deidara's battle with Sasuke Uchiha, Tobi provides support, helping Deidara seemingly by accident on multiple occasions, and later laying mines for him with a doton. After Deidara starts using C4, Tobi flees but is still in the area when Deidara's suicide attack is released. Zetsu reports that Tobi, as well as Sasuke Uchiha and Deidara have all been killed, however Tobi has escaped the explosion and traveled to the Rain village to meet with Pein (and Sasuke has escaped as well). During the meeting with Pein, Tobi refers to himself as Madara Uchiha and tells Pein to personally capture Naruto Uzumaki. Later on, Jiraiya reveals that Madara Uchiha is the ninja that battled and was defeated by the 1st Hokage at the Valley of the End. It was speculated by Jiraiya that Madara may possess the ability to summon the Nine-Tailed Demon Fox, a beasts said to come where human malice appears and grows, and lured it to attack Konoha. Jiraiya suggests that the demon fox was being controlled when it attacked Konoha at the time of Naruto's birth, and brings up Madara as the only person known to be capable of this.

Past Speculation
Tobi never actually revealed himself to be Madara Uchiha, he only said that he has the power of Madara Uchiha which many times people said they have the power of someone, but they were really they were just trained by them, were related to them, or compaired there power to them.

Another fan speculation which is most likely is that Pein, who now is discovered to have more bodies could have used the remains of Obito to reconstruct his body and revive Madara in the body which contained one Sharingan. It is also possible that he employed Kakuzu, who was able to reattach Deidara's arms after they were severed, and could have done similarily with Obito and Madara.

There are also speculations that Madara Uchiha is the third Uchiha clan member with the ability to possess the Mangekyo Sharingan as mentioned by Itachi Uchiha.