"Marume Nagayoshi is known as the man who created the Taisha-ryuu (or Taisha Style) of swordsmanship. Born in 1540, he had various exploits about Japan during the Sengoku and early Edo periods. He would later retire, and become a teacher of the arts--reading, poetry, etc.
In this game, Marume is working for the Tokugawa forces as a swordsmanship instructor. When he meets Miyamoto Musashi 宮本武蔵, he is recruiting men for a secret mission. He is impressed with Musashi, who is teaching students, and who is showing them a way to use both swords at once. Musashi tells them that this is only for show; just something to help get your mind focused. But, Marume believes that Musashi actually believes this could be a real fighting style. Marume convinces Musashi to join, and allows him to volunteer for a key role in the secret mission. He does, however, pause to ask Musashi to survive the ordeal.
It turns out that it was a set-up, designed by the advisor, Nankoubou Tenkai 南光坊天海, and that the two would be hunted down and killed at the meeting point as if they were murderers. Presumably, the rest of Marume's team was sent somewhere else or killed, too. Marume is defeated by Musashi in the battle at the temple they were supposed to meet up at, but another key figure in the treachery at hand, Sasaki Kojirou 佐々木小次郎, shows up to ensure his capture (death). Musashi tells Marume that he will take his advice and survive, and flees.
However, Marume is not completely an evil figure. He aids Musashi, luring him to compete in the tournaments at the Houzouin Temple (really the Koufuku Temple in Nara) by spreading a rumor that an imposter would compete under Musashi's name. When Musashi meets Marume at the temple, he alludes to the fact that there are other, more sinister forces at work, and that he is on the same side as Musashi in that he, too, wants to see Haruka protected.
When Musashi later visits the Yagyuu village to meet with their patriarch, Yagyuu Sekishuusai 柳生石舟斎 , he finds that Marume has been watching him. Although Marume has followed some of Tenkai's and Kojirou's orders, he hasn't been 100% faithful to them. He was sent to kill Musashi when the bounty hunters came to Ukiyo's 浮世 house, but, just took his sword instead. Later, when he is sent to kill Haruka's adoptive family, he spares her life, allowing her to escape.
Marume and Sekishuusai have been planning to stop Tenkai and to thwart Kojirou's blind ambitions. To do so, they needed a strong warrior like Musashi. Marume also obviously feels responsible for Musashi's position and seems to express regret over the treachery that left him a wanted man. In the end, Marume shows Musashi how to overcome the Shinkage-ryuu style of swordplay used by Kojirou in hopes that he will defeat him."