Kotetsu is a tsugumomo, a spirit born from a Japanese sword that has gained a soul through generations of use and human will. He manifests specifically as a katana, and his human form is that of a young boy with large blue eyes and short, sandy-blond hair that flips outward at the ends. Unlike many other tsugumomo who take on adolescent or adult appearances, Kotetsu consistently retains this youthful look.

He is currently bound to the exorcist Sunao Sumeragi, but before her, his previous owner was Suou Sumeragi. This history has deeply shaped Kotetsu’s personality. He possesses a quiet, hesitant demeanor, and years of unfair treatment at the hands of Sunao have left him feeling unwanted and mistrusted. This sense of being undervalued mirrors the early struggles experienced by Kazuya Kagami, the series' protagonist. Despite his subdued nature, Kotetsu is a powerful tsugumomo and is notably wary of Kiriha, another prominent tsugumomo in the story.

Kotetsu’s primary role in the narrative is that of a weapon and companion to Sunao Sumeragi. He supports her in her duties as an exorcist, and his abilities are strongest when used in sync with her. In her hands, Kotetsu can unleash his most devastating attacks, making them a formidable pair in battle. The bond between Kotetsu and Sunao is a central aspect of his character; they have forged a very strong relationship, though it is built on a foundation of initial mistreatment and gradual trust. Over the course of the story, Kotetsu’s development is tied to this relationship, as he learns to be valued and to trust his wielder.

As a sword-type tsugumomo, Kotetsu’s notable abilities are those of a powerful melee weapon. He is capable of cutting through supernatural threats and, when his full power is channeled, can produce immense destructive force. His fighting style complements Sunao’s aggressive exorcism techniques, allowing her to deliver lethal blows against amasogi and other hostile spirits.