Amidamaru is a 600-year-old samurai spirit born in feudal Japan on January 6, 1385. Orphaned young, he grew up among other orphans, including his closest friend Mosuke, the son of a swordsmith. Amidamaru trained in swordsmanship to defend the group from bandits, though his poverty meant he often broke poor-quality weapons. Mosuke later forged the durable katana Harusame (Spring Rain) from his father’s last hunting knife, which became Amidamaru’s signature blade. Both eventually served a feudal lord. When the lord, fearing Mosuke might forge a superior sword, ordered Amidamaru to kill him, Amidamaru refused. Instead, he helped Mosuke flee, promising to reunite on Funbari Hill. The lord’s spies discovered their plan, leading to an ambush in which Amidamaru fought without Harusame, killed many attackers, and died from exhaustion. Mosuke arrived too late, and both perished; Mosuke’s spirit haunted the sword for centuries.

As a spirit, Amidamaru waited for centuries at Funbari Hill for Mosuke’s return. He encountered Yoh Asakura after Yoh stopped delinquents from desecrating his grave. Initially refusing to become Yoh’s guardian ghost, Amidamaru agreed after Yoh facilitated his reunion with Mosuke’s spirit. Amidamaru then served as Yoh’s primary spirit ally, wielding Harusame throughout the Shaman Fight tournament. Their partnership evolved through multiple Over Soul forms, including O.S. Amidamaru, which fused Harusame with Amidamaru’s spirit into an expanding blade, Big O.S. for near-complete materialization of Amidamaru’s upper body, and O.S. Spirit of Sword, a colossal energy blade. His techniques included Shinkū Budda Giri (Vacuum Buddha Slash), a shockwave attack from his lifetime, and Gokōjin (Halo Blade), a rapid dual-sword strike. Amidamaru exhibited unwavering loyalty and stoicism, addressing Yoh as “Lord Yoh.” He reconciled with Tokagero, a bandit he killed centuries prior who later became an ally to Ryu Umemiya.

In Shaman King: Flowers, Amidamaru becomes the guardian spirit of Hana Asakura, Yoh and Anna Kyoyama’s son. He adapts to Hana’s more rebellious and battle-obsessed personality, mediating his conflicts, sympathetically addressing his frustrations, and reluctantly aiding in street fights. During the Flower of Maize events, Amidamaru was temporarily petrified, stolen, and later returned. Across all his appearances, he remains defined by combat mastery, a deep sense of duty, and the bonds forged in life and death. His name derives from Amitābha Buddha.