Lifespan: Unknown to 1/14 of Genki 3 (1572)
Name Changes: Takamasa → Kagessai Sōkei (Buddhist name)
Title: Governor of Mino
Clan: Okamoto (an illegitimate branch of the Haga clan)
Lord: Utsunomiya Hisatsuna → Utsunomiya Hirotsuna
Okamoto Sōkei served as a bushō during the Sengoku period.
Sōkei was a retainer of the Utsunomiya clan, the sengoku daimyō of Shimotsuke Province. The Okamoto were an illegitimate branch of the Haga clan, the mainstay of the kisei-ryōtō, the band of bushi serving as the elite soldiers of the Utsunomiya clan.
In the era of Utsunomiya Hisatsuna, Sōkei is deemed to have already been a retainer of the Utsunomiya clan, and, in the era of Utsunomiya Hirotsuna, he became the head of chief retainers of the Utsunomiya, wielding significant influence in the family. Sōkei was entrusted with managing diplomacy with Uesugi Kenshin of Echigo Province, cultivating intimate relations with Kenshin. In his later years, Sōkei’s son, Okamoto Takanaga, engaged in diplomacy with Kenshin and Oyama Hidetsuna (a sengoku daimyō and the eighteenth head of the Oyama clan).
Further, there is a letter addressed to the Sōchi clan stating that Sōkei was delegated governance of the Kurishima township formerly managed by Nagayama Tadayoshi (a close associate of Utsunomiya Tadatsuna) in addition to Haga Takataka and Haga Takatsune (leading figures among the senior retainers during the eras of Utsunomiya Okitsuna and Utsunomiya Hisatsuna).
The alliance between Takeda Shingen and Hōjō Ujiyasu known as the Alliance between Kai and Sagami Provinces raised fears among members of the Utsunomiya clan. Minagawa Toshimune, a senior retainer of the Utsunomiya who sought to pivot the Utsunomiya in support of the Gohōjō, viewed Sōkei, as the head of the faction supporting the Uesugi, to be an impediment to his designs. On 1/14 of Genki 3 (1572), in the evening after Toshimune arrived for service at Utsunomiya Castle, Sōkei was murdered. The next day, Utsunomiya Castle was occupied. This event is known as the Revolt of Minagawa Toshimune.
References to Okamoto Chikugo-Shōyū 秀卓 as the lord of Manago Castle in Shimotsuke appear, based on the similarity of the official title, to be in regard to Sōkei’s son, Takanaga (Chikugo-no-kami) while Okamoto 秀候, Okamoto Takamori (affiliated with Minagawa Hiroteru), and Okamoto 高候 (a hatamoto serving Tokugawa Hidetada in the early Edo period) are surmised to be descendants of Takanaga.