Being a product of Wano Country's superior craftsmanship, Soto Muso is a high quality sword. Combined with Kawamatsu's strength and skills, Soto Muso is powerful enough to defeat large numbers of Beast Pirates in a single strike.
Kawamatsu first bought Soto Muso during his youth when Yasuie gave him and his fellow retainers money to groom themselves. The sword then became his weapon as a samurai. Soto Muso was used by Kawamatsu in his Kappa-ryu swordsmanship, as Kawamatsu would later used Soto Muso to fight as Oden's retainer including their first battle against the Beasts Pirates twenty years ago. Soto Muso was later confiscated when Kawamatsu was captured alongside the Nine Red Scabbards and imprisoned in the Rasetsu Town. After they escaped with Oden's self sacrifice, Shinobu managed to retrieve their weapons including Soto Muso, which was given back to Kawamatsu. When Kawamatsu was caught stealing and imprisoned in Udon thirteen years ago, Soto Muso was again confiscated and separated from its master for seven years until Raizo found Kawamatsu by coincidence. Raizo later stole his cuff keys and Soto Muso and returned the sword back to Kawamatsu, allowing him to quickly defend himself.