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Izuna Uchiha is the younger brother of Madara Uchiha. There are many theories that say Izuna is Tobi. Izuna was a member of the Uchiha clan.
Life[]
Little is known about Izuna's life. He gained Mangekyo Sharingan and then at some point either gave them to or had them taken by Madara. He then died shortly before the founding of Konohagakure.
Tobi theory[]
Some claim that Izuna gave up his eyes to Madara, then took the eyes of another Uchiha. When he found out Konoha would be formed, he made a plan with Madara, then faked his death and disappeared. Years later, he transplanted Madara's eyes into Nagato, and took on the alias "Tobi". Later found out that Tobi was Obito Uchiha, so the theory is not correct,dumbasses