Hosokawa Katsuyuki served as a bushi during the late Muromachi period.
Katsuyuki was a yūshi, or a child raised by a family as one’s own. Originating from the Hosokawa-Yashū family, Katsuyuki was raised by Hosokawa Katsumoto, the supreme commander of the Eastern Army for the Ōnin-Bunmei War and eleventh head of the Hosokawa-Keichō family – the main branch of the Hosokawa clan. Katsuyuki’s natural father was Hosokawa Noriharu, a second cousin of Katsumoto. Similar to his adoptive father, Katsuyuki is surmised to have received one of the characters in his name from Ashikaga Yoshikatsu, the seventh shōgun of the Muromachi bakufu.
During the Ōnin-Bunmei War, Katsuyuki fought valiantly as a bushō in the Eastern Army under the command of Katsumoto. At the Battle of Shōkoku Temple, Katsuyuki, together with Yasutomi Mototsuna, led 3,000 soldiers to defend against an advance by the Western Army. This resulted in a bitter defeat to the opposition forces comprised of members of the Ōuchi, the Toki, the Hatakeyama, and the Isshiki clans. In the end, Katsuyuki and his troops withdrew. After incurring serious injuries, Katsuyuki was rescued by a retainer named Yuasa Munetake who sacrificed himself to enable Katsuyuki to flee to safety. Katsuyuki was moved by the loyalty displayed by Munetake. Later, he wed the younger sister of Munetake. The son born between them, Munemasa, was adopted by the Yuasa clan.
For a period of time, Katsuyuki was in a position to inherit the headship of the Hosokawa-Keichō family from Katsumoto but was opposed by retainers who emphasized his blood ties to the Yamana clan. In the first month of 1472, during an attempt to settle the war, Katsuyuki clashed with members of the faction within the Eastern Army who promoted peace with the Yamana clan. In 1473, after the death of Katsumoto, the headship of the Hosokawa-Keichō was inherited by Hosokawa Masamoto instead of Katsuyuki. Masamoto’s successor, Hosokawa Takakuni, was a nephew of Katsuyuki.