Tobi

, who poses as when first introduced, is a founder of the Uchiha clan, Konohagakure, and Akatsuki. When he joins Akatsuki as Tobi (replacing Sasori), he is partnered with Deidara, revealing himself as Madara after Deidara's death.

Background
Decades before the start of the series, Madara grew up in constant competition with his younger brother, Izuna Uchiha. Their competition led to both gaining the Mangekyo Sharingan, and with it they were able to take control of the Uchiha clan, Madara acting as its leader. Under his leadership the clan conquered all they came across, but overtime he began to lose his sight because of the Mangekyo Sharingan. To regain his vision he took his brother's eyes (an act Madara says his brother consented to), allowing him to continue to lead the Uchiha to prosperity.

In the years that followed, the Uchiha constantly clashed with the equally powerful Senju clan. To bring an end to the continued fighting, the leader of the Senju, Hashirama Senju, approached Madara with an offering of peace. Although Madara never wanted peace with the Senju, the rest of the Uchiha wanted to end the fighting and Madara had no choice but to go along with their decision. The Senju, the Uchiha, and all the clans they had conquered came together to form the village of Konohagakure. Against Madara's wishes, the villagers selected Hashirama as the village's First Hokage. Madara feared that Hashirama would oppress the Uchiha and tried to rally support for challenging his leadership. Rather than help him, however, the Uchiha turned their backs on him, believing his only motives were greed.

Abandoned by his clan, Madara left the village so that he could return to challenge Hashirama in battle. They fought at the site that would someday be called the Valley of the End where, although Madara called upon the Nine-Tailed Demon Fox, he was defeated and believed to have died. Madara faded into memory, and in time would found Akatsuki and become Kirigakure's Mizukage.

Twelve years before the start of the series, the Nine-Tailed Demon Fox attacked Konoha. Although Madara claims to have had no involvement in the attack, Minato Namikaze says the fox was under Madara's control at the time. A few years later he infiltrated Konoha to try and rekindle the flames of war. He was found by Itachi Uchiha first, who convinced Madara to spare the village in exchange for helping to wipe out the Uchiha clan for deserting him decades earlier. Madara complied, training Itachi and providing assistance. Itachi never truly trusted Madara, however, and kept an eye on him for the rest of his life.

Tobi
To keep in Akatsuki's shadow, Madara eventually took on the disguise of Tobi, giving orders to Pain in secret. He wears an orange, swirl-patterned mask that reveals only his right eye. He wore a different mask when he trained Itachi. His hair is cut back from the days of Konoha's founding and he wears a uniform consisting of a number of bolts and pins, which is usually concealed by his Akatsuki cloak.

While acting as Tobi, he is carefree and goofy, something that annoys most of the other members. Deidara, who believes all within Akatsuki should be serious and calm, is not pleased with Tobi's childish personality, and frequently attacks the latter in a comedic fashion when annoyed. Kisame and Zetsu, on the other hand, somewhat appreciate Tobi's ability to brighten up a gloomy organization such as theirs.

Personality
Madara exhibits a very calm and serious personality. He speaks with less formal diction, such as not using honorifics, and shows typically "villainous" arrogance. He speaks down to Pain and has great faith in his own abilities, only acting polite to Sasuke Uchiha. He will abandon his normal personality and favor of the Tobi persona when the situation calls for it, as he does when distracting a group of Konoha ninja.

Madara has shown great ability in manipulating others to do his work for him. Minato believes that he is using Pain's ideology against him in order to control him. He, similarly, befriends Sasuke in order to get him to join Akatsuki by telling him about Itachi's past, even though he has an apparent interest Sasuke and his abilities. While Akatsuki's goal to gain the tailed beasts is believed by most of the members to be a step in gaining control over the world, Madara implies that the real reason is to restore his Sharingan's power.

Rescue Gaara arc
After the death of Sasori, Madara, as Tobi, appears with Zetsu collecting Sasori's ring. He expresses interest in joining Akatsuki, believing that finding the ring will grant him membership. They then find Deidara's arm and the ring it wore, believing Deidara to be dead too. When Deidara appears and demands that Tobi give him his arm back, Tobi refuses, prompting Deidara to try and choke him to death.

Three-Tails Arrival arc
Tobi is accepted into Akatsuki as Sasori's replacement and Deidara's new partner. He is assigned to capture the Three-Tailed Beast, and Deidara accompanies him. In the anime, during their search, they stop to get a bite to eat at a dango shop, and Deidara watches anxiously for Tobi to remove his mask so that he can see what he looks like, but turns away hiding his face. When he finished his dango he mocked Deidara and his art and was promptly blasted away.

Later, after they find the beast, Tobi tries to convince Deidara to fight it in his place. The beast begins chasing him and Deidara uses one of his bombs on it while it's distracted. The rest of the battle goes unseen, but while they tow it away Tobi triumphantly claims to have defeated the Three-Tails in a single blow, though Deidara disagrees. Tobi later falls asleep atop the Three-Tails after making another cheeky comment, much to the anger of Deidara, who "wakes him up" by detonating a clay bomb next to him. They and the rest of Akatsuki seal away the Three-Tails and the Two-Tailed Monster Cat

Hunt for Itachi Uchiha arc
Before sealing the Four-Tailed Beast, Akatsuki learns Sasuke Uchiha has defeated Orochimaru. Once the sealing is completed Deidara, angry that he did not get to kill Orochimaru, brings Tobi with him in killing Sasuke instead. They find and confront Sasuke and Tobi is given the job of providing assistance for Deidara during the battle. The battle ultimately does not go in Deidara's favor, so he uses C0 in an effort to kill Sasuke. Zetsu reports to the remaining members of Akatsuki that Tobi was killed too, prompting Pain to remark that he is easily replaceable.

It is soon revealed, however, that Tobi lived, as he meets with Pain and Konan in Amegakure. He comments that Sasuke is developing nicely, and then instructs Pain to capture Naruto Uzumaki, the host of the demon fox. He later appears to distract the 8-Man Squad while Sasuke fights with Itachi. The Konoha ninja have no success striking him with their attacks, so Shino Aburame volunteers to fight Madara one-on-one. With his bugs, Shino is able to box-in and catch Madara, but Madara is able to escape from this as well. Zetsu then arrives to inform him that Sasuke has killed Itachi. Madara points out that he knew that would be the outcome and goes to collect Sasuke. Before leaving, his Sharingan was spotted by Kakashi.

Madara tends to Sasuke's wounds and, when he wakes up, offers to tell him about Itachi. Before he does so he starts to remove his mask as a sign of good-faith, only for Sasuke to use Amaterasu on him. In the shadows Madara is able to quell the flames, putting his mask back on before returning. He tells Sasuke that Itachi implanted Amaterasu within Sasuke to activate whenever he saw Madara's face, an effort to keep him safe and keep him from learning the truth. He tells Sasuke about the Uchiha clan's history, Itachi's secret life, and the lengths he had gone to to keep Sasuke and Konoha out of harm's way. Sasuke, regretting that he killed Itachi now that he knows the truth, resolves to destroy Konoha for forcing Itachi into the life he lived.

Sasuke & The Eight-Tailed Beast arc
Madara convinces Sasuke and his Team Hawk to start working with Akatsuki, the two groups having Konoha's destruction as a mutual goal. Before they can do that he states that Akatsuki needs the two remaining tailed beasts. Since Pain is still working on capturing Naruto, Madara sends Team Hawk to capture the Eight-Tailed Beast. After Hawk leaves Madara speaks with Zetsu, discussing Akatsuki's actions up to that point. He goes on to say that, although they've lost five talented ninja, all of their efforts have brought him closer to "making Sasuke his".

After Team Hawk captures the eight-tails' host, Sasuke gives it to Madara for Akatsuki to seal. During the extraction process it is revealed to be a fake. Madara is shown to remain silent, as Kisame comments that Sasuke has been fooled in the latter's attempt to capture the jinchūriki.

Abilities
Most of Madara's abilities are still unknown. He is remarked to be the strongest Uchiha to ever live and one of the most gifted ninja in history. He has also been noted to have been born with a particularly powerful chakra, which he states is the reason for his "stubborn refusal to die". Even the demon fox mentions that Madara's chakra is even more sinister than its own. One of the abilities he has actually displayed is his speed, which, as Tobi, was apparently his only claim to fame. To that end, Kisame comments "he is a good runner." Madara also appears to have immense strength, as he was able to stop a slash from Suigetsu's giant sword with a single arm.

He was armed with a giant scythe with a war fan attached to it during the time of Konoha's founding, fitting with his clan's name, which means "paper fan". He is also said to have been able to summon the Nine-Tailed Demon Fox. It is unclear if this referred to the use of the Summoning Technique or if he could produce a enough human malice to bring it to his location.

Mangekyo Sharingan
During Itachi's fight with Sasuke, it was revealed that Madara Uchiha and his younger brother, Izuna, were the first members of the Uchiha to activate the Mangekyo Sharingan, and used its power to take over the clan. After implanting his brother's eyes into his own body, Madara then awakened a new "eternal" Mangekyo Sharingan, a combination of his own and Izuna's Mangekyo Sharingan. It gave him access and mastery to his clan's most powerful jutsu, along with untold amounts of power without the risk of losing his eyesight. Madara's knowledge and mastery of this powerful Dojutsu allowed him to fight on par with Hashirama and it gave him the power to forcibly subdue the Nine-Tailed Fox. It was also thanks to Madara's knowledge of this Dojutsu that Itachi learned how to properly use his Mangekyo Sharingan.

Teleportation
Madara shows the ability to recover from debilitating or even 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. The same things occurs when Naruto hits him in the back with a Rasengan, where the attacks literally phase through him, avoiding damage.

Madara also shows the ability to travel long distances within relatively short time-spans, escaping the final explosion created by Deidara and reappearing in Amegakure within a very short period. It is speculated by Kakashi that he uses space/time Ninjutsu to instantly send himself or even parts of his body to different locations or some sort of void and instantly bring them back.

Past Speculation
Before Tobi was revealed to be Madara Uchiha, several fans hypothesized that he was Obito Uchiha, as his only shown eye was the opposite of the eye that was inserted in Kakashi. They also have similar personalities, hair styles, as well as the same blood type, "O". The spelling of Obito was also similar to Tobi, creating the fan reference of ObiTobi, or Tobito.

Trivia

 * The phrase "Tobi is a good boy" has proven to be very popular among the Naruto community. Although not said by Tobi himself (Zetsu's white half actually uttered the phrase to his black half after stating Tobi's intentions of joining Akatsuki wasn't so simple to accomplish), many have found the comment to have hilarious effect and as such, Tobi is depicted as a carefree child or a good natured character in many fan-made art and videos on Youtube while incorporating the phrase. Many have altered the phrase, after Tobi revealing that he's Madara, saying "Tobi is a Bad Boy".
 * Despite Tobi and Madara being the same person, the third Naruto databook gives them separate entries.
 * Although Itachi claims that Madara created Akatsuki in chapter 386, Pain says he created it in chapter 436.
 * can be translated as "jump", while can be translated as "spots".
 * The only members of Akatsuki who have been shown to know Tobi's identity are Pain, Konan, Itachi, Zetsu, and Kisame.
 * Madara has been described by Minato as having unfathomable power.