Madara Uchiha

Eighty years ago, when shinobi were battling as individual clans, not as villages, among a countless number of them, two clans emerged as the strongest: the Uchiha clan and the Senju clan. Madara was also the first to activate the Mangekyo Sharingan.
He had a younger brother, who was equal to him in every way and also awakened the Mangekyo Sharingan. Together they ruled over the Uchiha clan, with Madara taking the role as leader. But eventually Madara went blind, unaware of any side effects of the Mangekyo Sharingan at the time. He became increasingly desperate to regain his eyesight. In manga during the final fight of Itachi and Sasuke, it is revealed that Madara took away his brothers eyes which killed his brother. . At some point in Madara's early life, by the decision of the entire Uchiha clan except for him, he formed an alliance with the Senju clan, Hashirama Senju's clan, and together with its leader, the future First Hokage, Madara founded Konohagakure.
However, after Hashirama was chosen to be the First Hokage of Konoha, Madara feared that the Uchiha clan would lose supremacy and he sought the help of the Uchiha in his challenge for leadership. The Uchiha clan, however, refused, believing he was greedy. None of the Uchiha would help Madara, and he decided to leave the village. Feeling hurt and betrayed, he came back and wanted revenge against Konoha which led to a legendary battle at the Valley of the End against Hashirama. During the battle, Madara used his Mangekyo Sharingan to take control of the Nine-Tailed Demon Fox and used it in a desperate attempt to kill Hashirama. Madara, however, was ultimately defeated and was presumed by most to have died after the battle. Madara fled, and was never seen or heard of again out in the public for decades. To commemorate the battle, two statues, one of Madara and the other of the Hashirama, were carved into the earth around the waterfall. It is for this reason that Naruto Uzumaki and Sasuke's fight at that location was noted to be ironic.
Madara created the criminal organization known as the Akatsuki to look over problems. He hid behind the shadows and placed Pain as the figure-head leader, with Madara giving Pain orders from the background. Outside of Pain, the only members who are shown knowing Madara's position as the true leader are Konan, Kisame, Zetsu and Itachi.
Madara Uchiha prior to becoming Tobi.
Madara Uchiha prior to becoming Tobi.
Itachi believes it was Madara himself that controlled the demon fox to attack Konoha, though Madara denies this, saying that the demon fox was a natural disaster. Nevertheless, when he came to destroy the entire village of Konoha in revenge, he met up with Itachi, who had already been assigned his mission to kill his clan. Itachi successfully talked Madara into only killing the Uchiha clan as long as he left the rest of the village alone. Madara also taught Itachi about the Mangekyo Sharingan, helping him by teaching him how to unlock its true power. After Itachi killed off the Uchiha clan because of orders from Konoha, Madara let Itachi into Akatsuki for unknown reasons. Itachi refers to Madara today as "a pathetic shell of his former self, no longer worthy of being called the greatest Uchiha."
It has also been recently revealed that at some point after being presumably killed, Madara took the role of Mizukage.