Izuna Uchiha is Madara's younger brother. Not much is known about him except that he obtained the Mangenkyou Sharingan at the same time as his brother. Side by side, the brothers took over many Uchiha clans merging it under one banner. This shows that he is exceptionally skilled and maybe as skilled as Madara. When Madara's eyesight was beginning to fade, he gave up (or was taken from him) his eyes to his brother.