Uchiha Izuna is the younger brother of Uchiha Madara. He was killed by Senju Tobirama using the Flying Thunder God technique, which Tobirama specifically developed to counter the Sharingan. His death is a key event that deepened Madara's distrust of the Senju. Tobirama's ruthless tactics in killing Izuna are referenced as evidence of his suspicious nature. Izuna is not alive in the current timeline but his death is part of the backstory that defines Tobirama and Madara's relationship.
