Tobi is the principle antagonist of the series. He is a masked man whose identity has not been revealed. He was misled everyone to think he was Madara Uchiha so that he could instigate the Fourth Shinobi World War, but with Kabuto's revival of the real Madara he was found out.
According to Itachi Uchiha and the Fourth Hokage, Tobi instigated the Nine-Tailed Demon Fox's attack on Konoha sixteen years prior to the onset of Naruto's main plot, though Tobi denies this allegation. In any case, the Uchiha became an immediate target of accusations since the clan's ocular powers can control the fox. Subsequently, the clan was forced to relocate to a remote section of Konoha. Upon discovering that the clan planned a coup, Konoha's elders ordered the complete annihilation of the Uchiha clan, which was to be carried out by Itachi Uchiha. Tobi, who still held a grudge against the clan for betraying him, assisted Itachi on the mission on the condition that he didn't harm the rest of the village. Sometime later, Tobi joined Akatsuki and hid himself within the organization using an alias as a frivolous, clumsy ninja who wears a spiral patterned orange mask with one peep hole.
After Pain was defeated, it left the Akatsuki with just him, Zetsu, Kisame. After some time, Naruto and the 8-Tails, or Killer Bee, had to go into hiding after getting information that there was to be a ninja war to happen soon, so the Kage Summit agreed that Naruto and Killer Bee are to stay in hiding until further notice. Though, the Akatsuki's plan played out successfully and Kisame managed to have his sword, the Samehada, tag along with Killer Bee so that Kisame could hid inside of the sword to know the location of Killer Bee's and Naruto's Location. After Kisame had to resort to sacrificing himself in order to keep himself and the Akatsuki's information safe.
While that was happening, Tobi went after Konan to retrieve the Rinnegan eyes. Konan refused and challenged him to a battle. Konan planned out how the battle will be and made a Jutsu that can counter the Dimensional Teleportation Tobi can use. Luckily, Konan timed her paper bombs in order to hit Tobi and unleashed her new move, which is a justu involving paper bombs where there is an explosion for at least ten minutes, which can counter Tobi's Teleportation. Since Konan knew that Tobi's technique would have a five minute delay, she made that jutsu just for preventing Tobi from escaping.
Unfortunately, Tobi survived because Konan did not know of one of the special techniques that the Uchiha can use, Izanagi, the Forefather God, where the user has the ability to make attacks and such a dream or reality. With the technique being used, it costs the users vision to be lost forever, which is the reason why the Uchiha made that technique forbidden from use. Danzou took advantage of this technique by collection Sharingan eyes so that he would be able to use the technique once for each eye he had, with each a minute of delay.
After Killing Konan, Tobi then located the Rinnegan eyes and the location Nagato and Yahiko were laid to peace. Before retrieving the eyes, Konan's explosives cause part of Tobi's mask to break. So after the battle between the two, Tobi created a new mask. The new look of the mask appears to be still orange but has the Rinnegan ripple and the Sharingan commas positioned like the actual eye. At this point, with the information of Naruto's location, the Rinnegan eyes, and the Sharingan eyes, Tobi is now ready to head for Naruto to take the Nine-Tails.
During the war, Tobi was revealed to have the DNA cells of the First Hokage. It was speculated that he had implanted the DNA cells into his body making his body like Zetsu who was made through the first Hokage's cells. Tsunade says that this was probably how he had extended his life and that he is now immortal.
With the recent chapters, Kabuto was able to ressurect using the Edo Tensei the real Madara and Tobi was revealed to be a fake.
Tobi's most used jutsu is some kind of dimensional technique where he warps into his eye. But Tobi seems to be extremely skilled in Dimensional Techniques. He is able to dematerialize parts of his body and then materialize back making it seemed as though he is just an illusion when someone attempts to directly attack him.