Several hundred years ago, Izo was one of the founding members of the Hand. He was a close follower of the leader, Kagenobu Yoshioka.
Yoshioka was a blind martial artist, who used a radar sense to fight and "see" the world, and Izo blinded himself in order to see the world in the same way Yoshioka did and develop his radar sense. However, when Yoshioka died, Izo left the Hand because it was growing darker and more dangerous, becoming the Hand that is known today. In order to counter the Hand, he founded a rival martial arts group, known as the Chaste, but was soon thrown out by his students for his excessive drinking and gambling, which they found disgusting. Leadership of the Chaste was then given to Stick.
In the modern era, Izo was present when Matthew Murdock was struck by a chemical container, and reported the incident to Stick anonymously. He also trained the ninja Lady Bullseye, promising to make her leader of the Hand, but betrayed her and instead chose to support Daredevil in that endeavor, hoping that he might turn the Hand back towards the light.