Shigure Kosaka specializes in weapon-based martial arts. Her early life was spent isolated in the mountains with her father, a weapons craftsman for Yami. There, Sai Kagerou acquired a sword named Setsunamaru from her father and lethally tested it on a swallow, Shigure's first companion. Martial arts master Akisame Kōetsuji discovered them during his travels. Facing terminal illness, her unnamed father implored Akisame to grant him a warrior's death. Akisame initially declined, but a forced confrontation led to her father being thrown from a cliff. With his final words, he revealed his intent was to serve as a cautionary example, hoping to deter his daughter from repeating his mistakes of creating killing tools. Witnessing this, Shigure silently cried while clutching her father's final sword, Hachirobe. Akisame then named her "Shigure" (drizzle) due to the rain present and arranged her upbringing under an elderly Kōsaka-style weapons master. After this master's death, Akisame brought her to Ryōzanpaku.

Physically, Shigure has a slender yet curvaceous frame, fair skin, long dark hair usually in a high ponytail, light purple eyes, and distinctive lightning bolt-shaped eyebrows. She typically wears a short pink kimono, long purple stockings, bandages (sarashi) around her chest, and traditional fundoshi underwear. In serious combat, she dons chainmail secured by a black chest strap. She almost always carries her father's last creation, a tsuba-less nodachi with a bandage-wrapped hilt, strapped to her back.

Her personality is predominantly emotionless and quiet, often pausing between words. Initially solitary, she largely avoided interaction with other Ryōzanpaku residents except Apachai Hopachai. She holds deep respect and loyalty for Akisame, viewing him as a surrogate father and complying with his requests without hesitation. She exhibits a strong aversion to Kensei Ma's perverse behavior, often responding violently. Despite her stoicism, a feminine side surfaces occasionally, such as blushing when acknowledged as a woman by Akira Hongō or during captivity by Yami when stripped naked. She rarely smiles openly, though expressions of pride or happiness appear regarding Kenichi Shirahama's progress or his emulation of her fighting style. She harbors profound affection for animals, sharing a close bond with her pet mouse, Tōchūmaru, and reacts fiercely to harm directed at him. She experiences sleepwalking episodes accompanied by unsettling mutterings, unsettling masters like Shio Sakaki.

Her central philosophy is that weapons are extensions of the wielder's body; true mastery cannot rely solely on the weapon's inherent power. This aligns with the Katsujinken principle (life-saving fist). She frequently disarms or incapacitates opponents non-lethally, often by precisely tearing their clothing – a tactic effective against female adversaries who halt fighting to cover themselves. Even against formidable foes, she prefers using the back of her sword or inflicting only minor, non-fatal wounds.

Her character development emerged after Kenichi Shirahama arrived at Ryōzanpaku. She gradually opened up, participating in communal activities like shared meals. She displays protectiveness towards Kenichi, allowing Akisame to disclose her past specifically to him, stating it is acceptable "because he's Kenichi." She demonstrates candidness around him, bathing nude beside him without discomfort and suggesting repeated joint bathing sessions. Though she trains Kenichi less than other masters, she subtly yearns to instruct him more, often attempting to persuade him despite his fear of weapons. A significant moment occurred when she successfully offered Kenichi crucial advice during a mission, celebrating internally with a triumphant fist pump. She exhibits a near-total lack of conventional modesty, often causing discomfort in others, such as Kenichi suffering nosebleeds. Instances include fighting opponents naked, fishing nude, and undressing without embarrassment. However, she showed some modesty when naked in front of Yami researchers, potentially linked to simultaneous weapon confiscation. She undertakes independent missions to locate and retrieve swords forged by her father, removing them from circulation; her precise motivations remain private but are strongly implied to prevent their use for killing.

Her martial prowess is formidable. As Ryōzanpaku's youngest master and successor to the Kōsaka-Style, she holds the titles "The Prodigy of Sword" and "Mistress of all Weaponry." She exhibits extraordinary mastery over every conceivable weapon type, including swords, throwing knives, shuriken, chains, ropes, and tonfas, wielding even mundane objects like wooden spoons lethally. She conceals numerous weapons seamlessly. She attained the highest swordsmanship level, enabling her to "fuse" with her blade. She demonstrates remarkable combat efficiency, engaging multiple armed opponents simultaneously, including fighting seven masters of Yami's Hachiō Danzaiba single-handedly to facilitate Kenichi and Miu Fūrinji's escape. Her innate talent is exceptional; she displayed remarkable athleticism and weapon affinity from early childhood, quickly mastering the Kōsaka-style. She possesses immense physical attributes: supersonic attack speed allowing deflection of machine gun fire and overwhelming masters before movement perception; extraordinary agility enabling wall, ceiling, or rooftop traversal; immense strength permitting severing a tank's cannon with her sword and jamming it into the hull barehanded. Her stealth capabilities are masterful, allowing undetected appearances and effective presence masking, making her a potent infiltrator or potential assassin. She is also a master blacksmith, inheriting her father's skills. She possesses extensive knowledge of complex metalworking techniques, forging high-quality weapons and protective gear from raw materials, including creating iron sand and utilizing traditional Japanese sword-making methods. This expertise made her a target for Yami, who imprisoned her and stripped her of materials, fearing her ability to weaponize anything. She overpowered guards using minimal tools, leading them to call her a "witch." Her combat skills extend to proficient hand-to-hand combat, defeating restrained guards using kicks and punches during her Yami captivity.