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. Only Konan and Pein who were the only Akatsuki members known to have been present at that meeting, Zetsu, and apparently Itachi Uchiha know his true identity. He is one of three people who can use the Mangekyo Sharingan, the other two being Itachi Uchiha, and Kakashi Hatake.

Background
SPECULATION IS ABOUNDING IN THIS ENTIRE ARTICLE! NEVER STATES THAT TOBI IS MADARA IN MANGA! Madara Uchiha is the "founding father" of the Uchiha clan. He was also the first to activate the Mangekyo Sharingan. He had a younger brother, who was equal to him in every way, even possessing the Mangekyo Sharingan, and together they ruled over Uchiha. But eventually Madara's eyes descended into darkness. He did everything he could, but nothing worked. At the bottom of the darkness, he was possessed by the Mangekyo Sharingan. He killed his brother and stole his eyes to activate a new Mangekyo Sharingan that never faded, thus giving him unprecendented power.

He had an alliance with the Senju Clan of the Forest, the First Hokage's clan, and with its leader, the First Hokage, Madara founded Konohagakure. They fought over the laws that should be made in the new village, which led to a battle at the Valley of the End, the battle ending with Madara's defeat, but not death. Madara fled, creating the group Akatsuki. To commemorate the battle, two statues, of Madara and the First Hokage, were carved into the earth around the waterfall. It is for this reason that Naruto Uzumaki and Sasuke Uchiha's fight at that location was noted to be ironic.

Madara created Akatsuki to look over problems. He hid behind the shadows and placed Pein as the figure-head leader, with Madara giving Pein orders from the background. Outside of Pein, the only members who are shown knowing Madara's position as the true leader are Konan, Zetsu, and Itachi.

It has been revealed that Madara did in fact control the demon fox and used it during the attack on Konoha. It remembers and hates Madara for controlling him, and he compares Sasuke's eyes to be as evil as Madara's.

Madara's failure to unleash the demon fox to destroy Konoha made him give up, and so he taught Itachi about the Mangekyo Sharingan and helped him to destroy his entire clan, also teaching him how to unlock its true power. After they killed off the clan, Madara let Itachi into Akatsuki. Itachi refers to Madara today as "a pathetic shell of his former self, no longer worthy of becoming the greatest Uchiha."

Personality
Madara is very calm and serious, a trait he shares with most of the Akatsuki. He speaks with less formal diction, such as the using honorifics, and shows typically 'villainous' arrogance. Despite this apparent arrogance, he is not above complimenting the abilities of Sasuke Uchiha and Naruto Uzumaki, if only for the interest of Sasuke's Sharingan and Naruto's tailed beast.

Tobi
To keep in Akatsuki's shadow, Madara eventually took on the guise of Tobi and was placed within Akatsuki as Zetsu's subordinate. As Tobi, Madara wears an orange mask that covers his entire face outside of his right eye. Furthermore, Tobi has several bolts or pins present on the arms of his uniform, obscured by his Akatsuki cloak. While acting as Tobi, unlike others within Akatsuki, Madara is carefree and goofy. While Tobi, Madara also uses a very formal and correct manner of speech.

While Tobi greatly respects his other members, referring to Deidara as Deidara-senpai in the Japanese version, most of the other members tend to get easily annoyed by Tobi. Deidara, who believes all within Akatsuki should be serious and calm, is not pleased with Tobi's childish personality, and frequently attacks Tobi in a comedic fashion when annoyed. Kisame, on the other hand, somewhat appreciates Tobi's ability to brighten up a gloomy organization such as theirs. Ironically, Kisame is also the only living Akatsuki member who doesn't know Tobi's true identity.

As Tobi, Madara has not been seen wearing a forehead protector, probably to seal his identity, though he already has a mask covering his forehead, so this could also be why it is not visible.

Abilities
Madara is known to have the Sharingan and Mangekyo Sharingan, but the rest of his jutsu arsenal remains unknown, as he has never been shown fighting or utilizing jutsu. However, since the Uchiha clan is known for its fire techniques and since he is its founder, it is very possible he knows advanced fire techniques. He was armed with a giant fan which he most likely used to increase his fire techniques. He and Deidara were able to defeat the Three Tails, but Madara (as Tobi) was never shown doing anything but running from it. After the battle, which goes largely unwitnessed, he 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. While as Tobi, Madara is also shown planting Deidara's clay minefields underground during a later battle. Later it is shown that he became the "invincible immortal" that Itachi claims him to be by first killing his best friend to achieve the Mangekyo Sharingan and then made this permanent by taking the eyes of his younger brother.

Madara shows the ability to recover from what are presumably debilitating or fatal blows. After being slashed across the midsection by Sasuke's sword, he 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. The attacks literally phase through him, avoiding damage. Itachi later remarks Madara as an "invincible immortal.

Madara also shows the ability to travel long distances within relatively short timespans, escaping the final explosion created by Deidara and reappearing in Amegakure, then travelling to the Valley of the End all within a very short time span.

After he escaped Deidara's final explosion, it is revealed that he possesses the Sharingan, shown visible through the opening of his mask. The demon fox mentioned that Madara's chakra was sinister, more sinister than its own. Madara was the only person capable of taming the demon fox which was said to only come where human malice collects and festers, yet he was able to use the Sharingan to tame the tailed beast.

During Itachi's fight with Sasuke, it is revealed that Madara Uchiha and his younger brother were the first members of the Uchiha to activate the Mangekyo Sharingan, and used its power to take over the clan. After going blind he stole his brothers eyes and awakened a new "eternal" Mangekyo Sharingan, a combination of his own and his brother's Mangekyo Sharingan which gave him untold amounts of power without the risk of losing his eye sight. It was with this power Madara was able to forcibly subdue the demon fox.

Rescue Gaara arc
Madara is first introduced as Tobi alongside Zetsu collecting the rings of Deidara and Sasori. While Sasori's ring is collected, Deidara, who managed to live, appears and reclaims his own ring. After annoying Deidara three times, Deidara responds by comically choking Tobi with his feet. Using Sasori's ring, Tobi manages to join Akatsuki, though Zetsu claims isn't as easy as collecting the ring.

Hidan & Kakuzu arc
Tobi, who by now has successfully joined Akatsuki, is partnered with Deidara and sent to collect the Three-Tailed Beast. After Deidara reveals that this is Tobi's mission, Tobi, panicking, is seen only running away from the Three-Tails in a comic fashion. After the battle, which goes mostly unwitnessed, Tobi triumphantly claims to have defeated Three-Tails in a single blow, though Deidara disagrees. Tobi later falls asleep atop the Three-Tails, much to the anger of Deidara, who "wakes him up" by detonating clay next to him.

Current arc
Tobi is seen following Deidara, who is chasing Sasuke Uchiha, and eventually join Deidara's battle with Sasuke, though he does little besides laying mines for him and serving as a distraction. When Deidara prepares to use C4, Deidara orders Tobi to flee, though, while escaping C4, Tobi is later shown getting killed by Deidara's final explosion.

It is later revealed, however, that Madara lived, as he is seen ordering Pein in Amegakure just a few minutes after Deidara's death. Madara gives Pein the order to personally capture Naruto Uzumaki. Madara then travels to the Valley of the End on his own statue as Tobi, and while resting there comments that Sasuke Uchiha's Sharingan is growing more powerful.

Madara appears as Tobi to stop Naruto, Hinata, Yamato, Kakashi, Sakura, Shino, Sai, and Kiba from searching for Sasuke and Itachi Uchiha, and phases through all attacks the group sends, even a full-powered Rasengan from Naruto Uzumaki.

Past Speculation
Before Tobi is revealed to be Madara Uchiha, several fans hypothesized that he could be Obito Uchiha, as his only shown eye was the opposite of the eye that was inserted in Kakashi, which replace Kakashi's destroyed left eye in the manga-exclusive Kakashi Gaiden, and their similar hair styles. Also, they both act in the same goofy way, and Tobi has claimed to be a good boy, while Obito was a good boy. The spelling of Obito was also similar to Tobi, creating the fan reference of ObiTobi, or Tobito.

Another fan speculation is that Pein, who now is discovered to have multiple bodies, could have used the remains of Obito to reconstruct the body and revive Madara in the body of Obito, which contained Sharingan only within the right eye, the same eye that Madara has been shown using the Sharingan. It is also possible that Pein employed Kakuzu, who was able to reattach Deidara's arms after they were severed, and could have done similarly with Obito and Madara.

One more speculation is that Orochimaru, who was able to use the Summoning: Impure World Resurrection, as demonstrated in the fight with the Third Hokage, and who was once a member of Akatsuki, probably used or learned it there. And so, it is possible that the body of Obito has been reanimated and the spirit of Madara has been implanted into him using another forbidden jutsu.