Tosa Domain
Hayashi Yūzō was a native of Tosa Province (modern-day Kōchi Prefecture), where his father, Iwamura Hidetoshi, was a samurai in the service of Tosa Domain. His elder brother was Iwamura Michitoshi and his younger brother was Iwamura Takatoshi, both of whom served in numerous posts within the Meiji government. He was adopted into the Hayashi family at an early age. During the Boshin War, he fought against the forces of the Tokugawa shogunate in Echigo Province.