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
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. As stated by Kishimoto on Databook 3, Izuna died in battle.

Abilities
As the younger brother of Madara Uchiha, the strongest of the clan, Izuna himself was said to have been equal to his brother in every way.

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

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

Behind the Scenes
As Izuna Uchiha was said to be Madara Uchiha's equal in every way, it is unlikely his eyes were stolen by a blinded Madara. Izuna and Madara both gained the Mangekyo Sharingan at the same time, yet Madara went blind but not Izuna. This is most likely due to Izuna used his eyes less than Madara, therefore, Izuna had yet to suffer the complete blindness.

Trivia

 * He looks a lot like Sasuke and like Sasuke, he is also the younger brother, and has a couple of similar qualities.