Shigure Kōsaka is the weapons master of the Ryozanpaku dojo, recognized as the youngest and only female master among its resident instructors. She spent her early childhood in isolation deep in the mountains with her father, a master blacksmith and weapons wielder. After her father developed a terminal illness, he forced a final duel with Akisame Kōetsuji, seeking a warrior's death. Akisame reluctantly fulfilled the request, then gave Shigure her name, as her father had never given her one, and took her into his care. She was later formally trained in the Kosaka style of weaponry by a successor master and eventually joined Ryozanpaku after his death.

Shigure is defined by her deeply mysterious and stoic nature. She speaks very rarely and often in a neutral, flat tone, preferring the solitude of the dojo rafters to the company of others. This quiet demeanor hides a contemplative mind and a well of sorrow stemming from her father's death and the violent legacy of the weapons he forged. Her primary driving motivation is a solemn quest to locate and take out of circulation every blade her father ever crafted, driven by a desire to prevent them from being used for killing.

Within the narrative, Shigure serves as a mentor to the protagonist Kenichi Shirahama, though she trains him the least of all the masters. She focuses on overcoming his acute fear of weapons and teaches him practical skills such as swimming and rope tying. Despite her minimal direct training, her connection with Kenichi is profound and serves as the catalyst for her character development. His presence gradually draws her out of her isolation, and she begins to join the others for dinner, shows visible concern for his safety, and allows him to learn the truth about her past. Her key relationships include a paternal bond with Akisame, who raised her, a playful and childlike rapport with Apachai Hopachai, and a habit of physically punishing Kensei Ma for his intrusive behavior. She is constantly accompanied by a uniquely talented pet mouse named Tochumaru.

As a fighter, Shigure is a master of the Kosaka style, which grants her complete mastery over all forms of weaponry. She firmly believes a weapon must be treated as an extension of the user's body rather than a mere tool of destruction. Her fighting approach is that of a Sei-type, emphasizing calmness, lucidity, and precise control over raw aggression. Her skill is sufficient to effortlessly disarm and defeat armed groups of trained fighters. Over the course of the story, her development is defined by her movement away from solitude and grief. She evolves from a lonely, emotionally closed-off individual into a fully integrated and protective member of the Ryozanpaku family, finding a sense of belonging and emotional stability through the bonds she forms at the dojo.