Mitsuru Osafune is a slender young woman with ombré hair cascading to her knees, darkening at the roots before fading to a lighter tone. Her body bears several scars—one slicing across her nose, another etched into her chest, and a third marking her left thigh. She dresses in a bandeau and fundoshi beneath a loosely fastened kimono, a small sword ever-present at her side.

In the Daito Mine Arc, she presides over the Kitetsu Blades auction, instructing participants such as Musashi and Kojiro to choose swords through instinctive emotional bonds. She outlines the trial’s rules: blades resonate with the wielder’s soul color, accepting only those whose hues align. Five soul colors exist, each unlocking unique powers. When Musashi’s black soul causes his initial failure, she references ancient legends of such outcomes and urges him to forsake the path of a Bushi. A subsequent Kishin attack reveals his latent potential as his sword shatters under surging energy.

Later, she grants Musashi and Kojiro a second trial at her castle. Musashi’s success unveils a rare red soul, a feat she recognizes with solemn approval. Her guidance underscores deep expertise, though her own past remains shrouded.

Her combat prowess includes wielding both her compact blade and a Kitetsu Blade. The name "Mitsuru" may signify "fullness" or "beautiful crane," while "Osafune" nods to a historic swordsmith town in present-day Setouchi City.