Mosuke is Amidamaru's friend and he is the swordsmith who crafted Harusame, Amidamaru's sword.
Yoh and Manta first meet Mosuke at a museum when Mosuke is crying over the sword, Harusame. Mosuke blame himself for killing Amidamaru. However, that is not the truth.
Amidamaru and Mosuke are orphans whose parents died in the feudal war of Japan. They and other orphans have to survive by sticking together.
Young Mosuke and Amidamaru
In the manga, the past of Mosuke goes in more detail explaining what material Mosuke used to craft Harusame. Mosuke uses his father's knife to forge Harusame and Amidamaru is happy that Mosuke is able to forge the best sword. Some years later, Amidamaru and Mosuke present Harusame to a lord. The lord is impressed by the sword's beauty and accepts both Amidamaru and Mosuke. However, the lord wants Harusame to have a rarity value, so he instructs Amidamaru to kill his best friend so no one can forge a sword better than
Mosuke and Amidamaru at Lord's place
Harusame. Amidamaru talks to Mosuke after the incident and he refuses to kill him but to tell him to leave. Yet, Mosuke wants Amidamaru's sword, so he can forge it again. But, the feudal lord's men killed Mosuke and Amidamaru without Harusame face them. Amidamaru displays excellent swordsmanship. However, Amidamaru cannot fight forever and he is killed also.
Reaching his goal
Yoh helped Mosuke by integrating with him to forge Harusame and Yoh gives Harusame to Amidamaru. Mosuke did not wish to see Amidamaru, but he gives Amidamaru his best wishes.