Lifespan: xxxx to 10/7 of Tenbun 7 (1538)
Title: Assistant Captain of the Outer Palace Guards of the Right Division; Oyumi kubō
Clan: Ashikaga (Koga kubō of the Kantō region)
Father: Ashikaga Masauji
Siblings: Takamoto, Yoshiaki, Motoyori, others
Children: Yoshizumi, Yorizumi, 雪下等覚院某, Shōgakuni, daughter (wife of Uesugi Norihiro), Kyokuzan-ni
Ashikaga Yoshiaki served as a bushō during the late Muromachi period. A member of the Ashikaga clan, Yoshiaki was the son of Ashikaga Masauji, the second Koga kubō, and the younger brother of Ashikaga Takamoto, the third Koga kubō. Yoshiaki carried the title of the Oyumi kubō.
Yoshiaki entered the priesthood from an early age at the Tsurugaoka-Hachiman Shrine, adopting the name Kōnen, having the meaning of vacant or void. Owing to a confrontation between his father and older brother, Takamoto, in the Eishō Conflict, Kōnen moved to Shimotsuke Province and changed his name to Sōsai. After returning to secular life, he became known as Ashikaga Yoshiaki. With the support of Mariyatsu Nobuyasu, the lord of Mariyatsu Castle in Kazusa Province, Yoshiaki attacked Oyumi Castle in Shimōsa Province, defeating Hara Tanetaka, Hara Toratane, and Takagi Taneyoshi and capturing the castle. Yoshiaki then adopted the title of Oyumi kubō and opposed the Koga kubō. Following the death of Uesugi Tomoyoshi, lord of the Ōgigayatsu-Uesugi clan, Yoshiaki moved his base from Takayanagi to Oyumi in the summer of 1518.
Thereafter, he adopted a policy of opposition to Mariyatsu Nobuyasu. After the death of Nobuyasu, he intervened in an internal dispute within the Mariyatsu clan, banished Mariyatsu Nobutaka, and backed Mariyatsu Nobumasa as the new lord. Meanwhile, Nobutaka joined forces with Takamoto, his son (Ashikaga Haruuji), and the Gohōjo clan from Sagami Province to oppose Yoshiaki. In 1538, Yoshiaki raised an army, deployed to Kōnodai in Shimōsa Province, and engaged in a showdown with Hōjō Ujitsuna, a sengoku daimyō and the second head of the Gohōjō clan, in the First Battle of Kōnodai. Yoshiaki excelled in military strategy, serving himself as leader of the vanguard. He initially seized the advantage against the combined forces of Haruuji and Ujitsuna. However, forces under the command of Satomi Yoshitaka lacked the spirit to fight and gradually lost their gains. Yoshiaki’s younger brother, Ashikaga Motoyori, and his eldest son, Ashikaga Yoshizumi, died in battle. Upon hearing this news, Yoshiaki attacked headlong into Ujitsuna’s forces, dying in the counterattack.
Yoshiaki’s death led to the end of the role of the Oyumi kubō, but his second son, Ashikaga Yorizumi, continued to survive, first under the protection of Satomi Yoshitaka and Yoshihiro, then later under Toyotomi Hideyoshi.