Izuna Uchiha

was a shinobi of the Uchiha clan. He along with his brother: Madara Uchiha, were renowned as the clan's two strongest members.

Background
Before the era of ninja villages, Izuna was one of five brothers born to Tajima Uchiha. However, three of his brothers died at the hands of the Senju, leaving Izuna and his older brother Madara. Alongside Madara, Izuna was considered one of the most gifted members of the Uchiha. Izuna often competed with Madara as training. As a child growing up during the war-torn era, Izuna once accompanied his father on a mission to kill Hashirama Senju, whom Madara — albeit unknowingly — had been meeting. Once there, Izuna and his father were confronted by Butsuma and Tobirama Senju, the latter of whom he acknowledged by name.

As Izuna clashed with Tobirama, unbeknownst to him, Butsuma had launched an attack at him, hoping to strike him down and gain an advantage at defeating the evenly-matched Tajima. This attack, however, was deflected by his brother Madara who had intervened between the two sides along with Hashirama. Izuna later showed shock when Madara noted that Hashirama was stronger than he was, but was later elated to see that his brother had awakened his Sharingan.

As the battles between the two clans raged on, Izuna awakened his own Sharingan and became one of the first two Uchiha to awaken the Mangekyō Sharingan. With their newly obtained power, the two brothers became the heads of the clan, with Madara becoming its leader. During a battle between Hashirama and Tobirama however, Izuna was mortally wounded by Tobirama's technique. With Madara quickly rushing to Izuna's aid, Hashirama pleaded with Madara to come to peaceful terms. Seeing his brother begin to consider this offer, Izuna told his brother not to listen to their lies, ultimately making Madara retreat with Izuna.

When Madara later returned to the battlefield, he revealed that the injury inflicted on Izuna was fatal. According to Obito Uchiha years later, the dying Izuna had given Madara his eyes so his brother could obtain the Eternal Mangekyō Sharingan to protect their clan from its growing number of enemies and the Senju. Because of the blindness caused by overuse of a Mangekyō Sharingan, many people came to believe that Madara took Izuna's eyes by force to regain his sight.

Personality
Izuna Uchiha was a harmonic and devoted person. He also appeared to be very competitive with his brother, Madara, in terms of skill as they would often spar with one another in an attempt to hone these skills. Izuna looked up to Madara greatly and was even in disbelief when as children, Madara admitted to being weaker than Hashirama. Likewise, Izuna was elated when Madara awakened his Sharingan shortly after. As he grew older, Izuna's love for his clan grew and he fought fiercely to protect it. He soon came to share his clan's opinion regarding the Senju clan, even asking Madara, while severely wounded, not to fall for Hashirama's "false" truce proposals. He looked up to his older brother and was so close to Madara that he even gave him his eyes as a dying gift as a means of ensuring that Madara will be able to better protect the clan's future.

Because of the Sharingan's ability to see past any attempt to blind the user, Izuna felt pity for those who did not possess one.

Due to having fallen victim to the Curse of Hatred, Izuna was unable to see the greater good outside of his clan. When Hashirama offered to have their clans join forces to eliminate the non-stop wars, Izuna went to his grave telling his brother not to trust the Senju as it would mean the demise of their own clan.

Appearance
Izuna was a fair-skinned man with spiky, black hair that had a blue tint to it and was tied in a ponytail. His bangs parted down the centre of his face to frame both sides of it, He wore the standard Uchiha outfit which consisted of a high-collared, blue shirt with the Uchiha crest on his upper back and blue pants which were wrapped up in bandages at the ends. On his waist, he had a bandage belt that held a sack which presumably held ninja tools. As a child, his ponytail was absent, and his hair was spiky in the back, and he wore the standard high-collared robes worn by other Uchiha of the Era of the Warring States with the Uchiha crest on the back, with an armoured chest plate. According to Hashirama Senju, Izuna bore striking resemblance to one of his clansmen, Sasuke.

Abilities
Izuna was noted to be a prodigious talent, even by the battle-adept Uchiha clan's standards, and one of the most powerful Uchiha in history. He and his brother competed daily to hone their skills. Both of them had exceptionally strong chakra, even for Uchiha members. His prowess he was able to fight on equal ground with Tobirama Senju, a shinobi renown as one of the strongest and the fastest of his time, and even take the upper hand for a short time.

Nature Transformation
In the anime, Izuna showed proficient skill in Fire Release, able to perform the Fire Release: Great Fireball Technique with enough power to disperse a Water Dragon Bullet Technique from Tobirama, a highly proficient Water Release user.

Kenjutsu
Izuna was skilled in kenjutsu, able to fight on par against a young Tobirama Senju. He could proficiently wield it backhanded or normal, letting him attack in different patterns. His way of holding the sword's scabbard hints that Izuna was a practitioner of the Iai fighting style.

Sharingan
Izuna awakened and mastered his Sharingan at a very young age; around the same time as Madara. His prowess with the dojutsu was such that his clan soon recognised his high level of abilities even amongst their battle-excelling clan. With it, Izuna gained access to the dōjutsu's various generic ability, including to see events with advanced detail and from it easily evade most enemy attacks. His Sharingan was highly formidable, making one such as Tobirama avoid making eye-contact in fear of its effects.

Mangekyō Sharingan
Along with his brother, Izuna became the first Uchiha to awakened the Mangekyō Sharingan. Its design had three straight bars stretching out from his enlarged pupil. While never seen using this form in battle or any of its acquired techniques, Izuna's new ocular abilities quickly rose him along with Madara to leadership roles within their clan.

Legacy
While posing as Madara, Obito Uchiha claimed that Izuna's death after giving Madara his eyes played a great role in Madara's antagonism towards the Senju clan when they proposed a truce. Madara would have thought Izuna's sacrifice to keep him strong and protect the clan as being in vain. Izuna's eyes continued to be of great importance, first, by granting Madara the Eternal Mangekyō Sharingan, and in his old age, Madara would also awaken the Rinnegan with Izuna's eyes. The same eyes were later transplanted into Nagato, who would later go on to become Pain, and a powerful figure-head of Akatsuki, and then into Obito Uchiha, who came to share Madara's ideals, and while posing as him, plunged the world into the Fourth Shinobi World War. Upon both Madara and Tobirama returning to the world of living, Madara expressed his grudge against the Second Hokage for killing his beloved brother, and relished in making him suffer in revenge despite being focused on proceeding with the Eye of the Moon Plan. Hashirama noted that Sasuke Uchiha reminded him of Izuna, and believed that Sasuke is the sole person capable of saving Madara from his path of vengeance and hatred.

Trivia

 * is the name of a fox spirit in Japan and of a mountain. The latter is usually called Mount Iizuna, though. Iizuna, written with the same kanji, is also the Japanese name for the least weasel. Izuna's name can also be from a mythical god and wind spirit named Izuna that presents himself in the form of a tengu, which may be a reference to Susanoo's perfect form that resembles a tengu.
 * Izuna and Obito share the same birthday, and blood type.
 * According to the databook(s):
 * Izuna's hobby was training with Madara.