"The elder of the Yoshioka brothers, and one of the heads of the famous Yoshioka Doujou. He is a master of the sword, while his younger brother is simply better at his public image.
His students conspire against him. A rumor is started in which Miyamoto Musashi 宮本武蔵, known as a criminal and one of the best swordsmen alive, will come to challenge the Doujou. Musashi, living incognito as a jack-of-all-trades named Kiryuu Kazumanosuke 桐生一馬之介, decides he needs to investigate to find out information about the impostor in order to fulfill his obligations to his client, a young girl named Haruka.
However, it is Seijuurou's student, Gion Touji 祗園藤次, who has orchestrated the duel, and uses it as a trap for Musashi, luring him out of hiding and forcing him to face Seijuurou. At the same time that Seijuurou will be fighting Musashi, the traitor will go to kill Seijuurou's older brother, Denshichirou.
Seijuurou knows what's going on, but can't resist the duel because he wants to face a stronger opponent as a swordsman. When he is bested, he tells Musashi that only a person who knew the location of the duel could have tricked him into meeting him there, so that person is the traitor Gion Touji 祗園藤次.
Various histories have various accounts of the incident between Musashi and the Yoshioka family. One of them mentions him mortally wounding a man named Yoshioka Seijuurou, but, it is unclear as to exactly who that is. There is a fencing instructor at that time, in 1604, named Yoshioka Naotsuna, but there is never a concrete link between that person and the one named in the histories. He has also appeared in the book, Miyamoto Musashi by Yoshikawa Eiji."