Birth year unknown. Believed to be from Echizen Province (present-day Fukui Prefecture), though much of his biography is legend. He studied Chujo-ryu (or Toda-ryu) swordsmanship and favored a long sword called 'Bizen Osafune Nagamitsu' (nicknamed 'Monohoshi-zao,' the Drying Pole). His secret technique 'Tsubame-gaeshi' (Swallow Reversal) was reportedly devised at Ichijodani Falls in Fukui by watching swallows in flight. He served as sword instructor to the Hosokawa clan and was renowned as the greatest swordsman of his day. On April 13, 1612, he dueled Miyamoto Musashi on the small island of Funa-jima (later called Ganryu-jima) in the Kanmon Straits. Irritated by Musashi's deliberate lateness, Kojiro drew his sword and threw away the scabbard. Musashi reportedly said: 'Kojiro, you have lost! Why would the victor throw away his scabbard?' Kojiro was struck down by Musashi's wooden sword carved from an oar and killed.

A proud, solitary swordsman with absolute confidence in his technique. However, the line between legend and historical fact is blurred, and his true character remains uncertain.

As Miyamoto Musashi's greatest rival, he is the other half of the most famous duel in Japanese swordsmanship history. The Ganryu Island duel has been depicted in novels, plays, and films for centuries. The name 'Tsubame-gaeshi' lives on in modern kendo, manga, and games.