Yagyuu Muneyoshi (who was called "Sekishuusai" by those who showed respect to him as elder in the family ruling the Yagyuu Domain) was a swordsman who died just one year after the game takes place. In his youth, it is said that he studied under Itou Ittousai 伊東一刀斎, but was defeated by Kamiizumi Nobutsuna, practitionare of the Shinkage-ryuu sword style, and his disciples.
Muneyoshi became a student of the style, and eventually would master it himself. He would call his particular style "Yagyuu Shinkage-ryuu." His sons, Yagyuu Munenori 柳生宗矩 and Muneaki, would become swordmasters for the shogunate.
In the game, Miyamoto Musashi 宮本武蔵 first encounters Sekishuusai as he flees from the Tokugawa forces after the treachery of the Battle at Sekigahara. Sekishuusai is disguised as a monk, and protects the exhausted Musashi from the pursuing soldiers. He then leads Musashi to the Gion 祇園 district and to Ageha 揚羽, sister of Ukiyo 浮世, the woman Musashi had failed to protect but who inspired him to again take up the sword to protect others. He also offers a new personal name for Musashi.
Much later, Musashi is told by the retired kubou, Tokugawa Ieyasu 徳川家康, to visit the Yagyuu domain and ask to see Sekishuusai. When Musashi finally does, he is shocked to find the old monk who helped him out years earlier. It turns out that Sekishuusai and Marume Nagayoshi 丸目長恵 have been watching Musashi and trying to lead him to protect Haruka 遥. Sekishuusai has one other request he makes of Musashi--to defeat his own son, Munenori, who has assumed the name Sasaki Kojirou, in combat and awaken him to his swordsman life.