Born in 1508 as head of the Kamiizumi clan, minor lords in Ōgo, Kōzuke Province (present-day Gunma Prefecture). Originally named Hidetsuna, he later took the name Nobutsuna. The Kamiizumi served Nagano Narimasa, lord of Minowa Castle, and Nobutsuna spent years fighting the Hōjō and Takeda clans. He studied the Kage-ryū sword tradition under Matsumoto Bizen-no-Kami, further researched Shintō-ryū, Nen-ryū, and other schools, and synthesized them into his own art: Shinkage-ryū. At its core lay the revolutionary concept of "Katsujin-ken" (the life-giving sword)—the idea that the sword exists not to kill but to preserve life. He also invented the "fukuro-shinai," a padded practice sword. Until then, training with wooden swords caused constant injuries and deaths; the fukuro-shinai made safe practice possible and greatly advanced the spread of swordsmanship. In 1563, when the Nagano clan was destroyed by Takeda Shingen, Nobutsuna abandoned his life as a warrior lord and devoted himself entirely to the sword. He traveled the provinces training disciples, and his encounter with Yagyū Muneyoshi (Sekishūsai) in Yamato became a turning point in the history of Japanese swordsmanship. Nobutsuna granted Muneyoshi the inka (full transmission) of Shinkage-ryū; Muneyoshi's son Munenori went on to establish the Yagyū Shinkage-ryū and serve as sword instructor to the Tokugawa shoguns. Nobutsuna is believed to have died in 1577, though details of his final years are unclear. He received the court title "Kamiizumi Ise-no-Kami" from Emperor Ōgimachi and is remembered as a "Sword Saint." Shinkage-ryū gave rise to many branches—including Yagyū Shinkage-ryū and Taisha-ryū—and forms a foundation of Japanese sword culture.