Musashi Asuka is introduced as a mercenary hired by the Yasha clan to fight on behalf of Seishikan Academy, making him a primary antagonist in the early part of the story. Unlike most other combatants in the series, he is not a trained ninja by birth; he is an ordinary person who possesses extraordinary natural fighting ability and later reveals latent psychic powers. His motivation stems from a deeply personal cause: his younger sister, Erina, suffers from a severe illness, and he becomes a mercenary in order to pay for her medical treatment. This personal stake drives his willingness to fight ruthlessly and without hesitation.

In terms of personality, Musashi Asuka is cold, detached, and relentless in battle. He shows no mercy toward his opponents and maintains a stoic, almost emotionless demeanor when facing enemies. Even among his allies in the Yasha clan and the Yasha Eight Generals, he remains isolated, with some members such as Kuroshishi openly expressing hostility toward him. His preference is to fight alone, earning him a reputation as a lone wolf. However, this icy exterior cracks when his sister is involved. In her presence, he becomes protective, gentle, and displays a caring side that contrasts sharply with his warrior persona. He also develops a competitive interest in Kojirō and Ryōma, acknowledging them as worthy foes.

His role in the story is that of the supreme obstacle for the Fūma clan during the Yasha arc. He single-handedly defeats several prominent Fūma warriors, including Reira and Kabutomaru, and critically wounds Kojirō early in the conflict, forcing him to retreat and recover. His overwhelming strength pushes the Fūma to their limits until Kojirō returns wielding the legendary blade Fūrinkazan. During his final battle with Kojirō, the psychic powers that previously gave him an advantage are neutralized by the sword, but he then reveals the sealed longsword that transforms into the sacred Golden Sword (Ougonken), marking the convergence of two legendary sacred blades. The fight concludes with the death of his sister, after which Musashi Asuka's reason for fighting dissolves, and his conflict with Kojirō ends.

Regarding key relationships, his bond with his sister Erina is the emotional core of his character. Every action he takes is ultimately for her sake. His rivalry with Kojirō is central to the story, evolving from pure enmity into a mutual respect between two powerful warriors. He also has a tense and distant relationship with the Yasha Eight Generals and Yasha Princess, who employ him but do not fully trust or befriend him.

In terms of development, Musashi Asuka begins as a mercenary driven solely by necessity, but his encounters with Kojirō and the loss of his sister lead to a shift. After the Yasha arc, he becomes one of the five Cosmo legitimate warriors in the Sacred Sword War arc, fighting against the Chaos group. In the later spin-off, Asuka Mumyouchou, his past before the events of the series is explored, including how he met his master, acquired the Golden Sword, and the circumstances surrounding his sister's death, further deepening his characterization.

His notable abilities include incredible physical combat prowess with a longsword-style wooden sword, and his signature technique is Hiryū Haōken (Flying Dragon Overlord Sword), a devastating thrust that once pierced through Kojirō's leg. He also possesses psychic powers, specifically telepathy and other psionic abilities, making him a psychic soldier (サイキック・ソルジャー). After the sacred sword is awakened, his wooden sword transforms into the Golden Sword (Ougonken), one of the ten legendary sacred blades, granting him even greater power.