Musashi is the Guardian Character of Kairi Sanjou, born from Kairi's dream of becoming stronger and protecting the weak. His name and appearance are modeled after a traditional samurai. He wears green samurai-style clothing and has a kendo sword strapped to his side. Musashi has short green hair and purple eyes, and he sports a chonmage, a traditional samurai hairstyle, as well as spectacles similar to those worn by Kairi.

In terms of personality, Musashi is level-headed, quick to respond, and reasonable, able to see more than one side of an argument. However, he is also known to harbor a strong dislike for women, which causes him to become irritated when Kairi shows romantic interest in Amu Hinamori. His primary motivation is to support Kairi's desire to become stronger and to protect those who are weak, reflecting the samurai ideals from which he was created.

Musashi's role in the story is to serve as Kairi's Guardian Character, aiding him in his duties and providing guidance. He is a supporting character who appears alongside Kairi during the events of the series, particularly when Kairi becomes a member of the school's Guardians. His key relationship is with Kairi, whom he accompanies and with whom he can perform a Character Change. During this change, Kairi gains a ponytail and a wooden katana, giving him a samurai-like appearance. When they undergo a full Character Transformation, they become known as Samurai Soul.

Musashi, like other Guardian Characters, has the ability to sense the presence of Heart's Eggs and other Guardian Characters. In the manga, he is shown using his sword to cut a pear, demonstrating his skill with a blade. His development is tied to Kairi's growth, as he supports and reflects Kairi's evolving sense of responsibility and strength.