Shigure Kōsaka, the youngest and sole female master at Ryōzanpaku dojo, possesses distinctive lightning bolt-shaped eyebrows, fair skin, long black hair usually in a high ponytail, and light purple eyes. Her typical attire is a small pink kimono worn over purple stockings, sarashi bindings, and a fundoshi. In serious combat, she adds chainmail secured by a black chest strap. She always carries her late father's tsuba-less nodachi, its hilt wrapped in bandages, strapped to her back.

Born nameless deep in the mountains to a master weaponsmith and wielder crafting blades for Yami while terminally ill, Shigure's life changed when Akisame Kōetsuji encountered them. Her father demanded a warrior's death duel; Akisame refused until forced to fight. Before dying, her father revealed his intent as a cautionary tale against his path. Akisame named her "Shigure" (drizzle) after the rain that day and secured her training under an elderly Kōsaka-style master. After that master's death, Akisame brought her to Ryōzanpaku.

Initially solitary, avoiding interaction except with Apachai, Shigure gradually opened up after Kenichi Shirahama's arrival. She joined group meals and showed subtle concern for Kenichi. Profoundly loyal to Akisame as a surrogate father, she obeys his requests unquestioningly. She exhibits rare feminine reactions, blushing when acknowledged as a woman by Akira Hongō or during Yami captivity. Her speech is deliberate, with pauses between words, and her demeanor typically emotionless, though she smiles proudly at Kenichi's progress or cheers his successful emulation of her combat style.

Philosophically, Shigure views weapons as body extensions, believing sole reliance on a weapon's power prevents true mastery. Adhering to Katsujinken (life-saving fist), she defeats opponents non-lethally, often tearing clothes or inflicting minor wounds, exploiting female opponents' modesty. She displays no personal modesty, appearing nude during bathing, fishing, or combat without embarrassment, though she reacted with discomfort when forcibly stripped by Yami. She harbors deep affection for animals, fiercely protecting her pet mouse Tōchūmaru.

Renowned as "The Prodigy of the Sword and Mistress of all Weaponry," Shigure mastered Kōsaka-style, wielding every weapon type—swords, knives, chains, even mundane objects like wooden spoons—with lethal efficiency. Her capabilities include wielding weapons with her feet, achieving "fusion" with her sword, and employing exceptional speed to overwhelm opponents instantly. She once severed a tank's cannon with her sword before manually jamming it into the hull. Her agility enables rooftop traversal and wall-scaling; her stealth allows undetectable infiltration. She fought seven masters of Yami's Hachiō Danzaiba simultaneously to aid Kenichi and Miu's escape, overwhelmed only by divided focus. Imprisoned by Yami, guards stripped her of all materials fearing her ability to weaponize anything, yet she overpowered them using environmental objects.

Shigure also inherited her father's blacksmithing expertise, forging protective gear using traditional sword-making techniques and refining metal from resources like iron sand. She undertakes missions to retrieve her father's scattered swords, impliedly preventing their misuse. While training Kenichi less frequently than other masters, she expresses disappointment at his reluctance toward weapons and takes pride in advising him effectively. Recurring interactions involve violently responding to Kensei Ma's inappropriate behavior and exhibiting unsettling sleepwalking tendencies noted by Sakaki.

Her development shows increased integration within Ryōzanpaku, candidness about her past with Kenichi, and implied romantic interest through actions like unconcerned nudity during shared baths or suggesting joint baths. She consistently prioritizes Kenichi's growth, triumphantly celebrating successfully guiding him during critical teaching moments.