Izuna Uchiha

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

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

Personality
Little is known about Izuna Uchiha's personality, but it can be assumed that he cared about the fate of his older brother and his clan enough to sacrifice his eyes and life for them (though the rest of the clan thought his sacrifice was murder committed by a greedy Madara). However, his willingness to kill his best friend to obtain the Mangekyo Sharingan shows a darker side of him.

Since Madara and Izuna were said to be equal, it stands to reason that it was impossible for Izuna to have his eyes forcibly stolen by a blinded and weakened Madara. Therefore, Izuna must have indeed gave them up willingly.

First Shinobi Wars
80 years prior to the start of the series, where shinobi clans were merely mercenaries and not villages, Izuna, together with his older brother, were the first to awaken the Mangekyo Sharingan, and because of that they became the leaders of the clan during their time. When his brother, Madara, became blind due to his Mangekyo Sharingan, Izuna willingly gave up his eyes to his brother (though the rest of the Uchiha clan thought Madara took them by force), but in turn died.

Abilities
Izuna Uchiha is said to be Madara Uchiha's equal in every way. They were both very gifted, even among the famous Uchiha clan. It is also said that both of the brothers awakened their Sharingan at a young age, and unlocked the Mangekyo Sharingan at the same time, though Izuna's Mangekyou Sharingan was the basis for the Eternal Mangekyou Sharingan.