Izuna Uchiha

was the younger brother of Madara Uchiha, who was known as the strongest Uchiha.

is the name of a wind spirit, and can be translated as "rice-rope".

Personality and Abilities
Little to nothing is known about Izuna Uchiha's personality but according to Madara, he cared about the fate of his older brother and his clan to the point he was willing to sacrifice his eyes and life for him (though the rest of the clan believed he was murdered by a desperate Madara). Izuna was said to be Madara's equal, both being powerful shinobi even among the Uchiha. Perhaps coincidentally, he greatly resembles Sasuke.

First Shinobi Wars
80 years prior to the start of the series, when shinobi were individual clans hired as mercenaries, Izuna, along with his older brother, were the first to awaken the Mangekyo Sharingan, and due to the power it gave them, Madara became leader of the Uchiha clan with Izuna at his side. According to Madara, when he became blind due to overuse of the Mangekyo, Izuna gave his own eyes to his brother so he could see again (though the rest of the clan believed Madara took them by force), but died as result. Some questions arise as to how he died by the loss of his eyes. With proper medical treatment he could have lived; he would have simply been blind. Perhaps for a Sharingan user, when both eyes are extracted, the owner dies. Some speculate that perhaps the clan was right in suspecting that Madara took his brother's eyes by force. As stated by Kishimoto on Databook 3, Izuna died in battle.

Mangekekyo Sharingan
Izuna Uchiha once possessed the Mangekyo Sharingan as well, his took the appearance of three bars stretching out of his pupil. It is said that he developed his around the time his brother Madara did.

Madara later pried these eyes out of his head (Madara claimed he gave them to him) thus creating Madara's Eternal Mangekyo, which took the appearance of Madara and Izuna's Mangekyo combined.

Behind the Scenes
As Izuna Uchiha was said to be Madara Uchiha's equal in every way, it stands to reason that it was unlikely his eyes were stolen by a blinded Madara thus giving Madara's explanation that Izuna willingly gave them up great credibility. His Mangekyo Sharingan seems to be different than the rest as he never went blind like the rest of the Mangekyo users.

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