The protagonist and a female samurai. She is a mixed-race Japanese person with blue eyes, born to a Japanese mother and a white father. Due to the social prejudices of the era, she disguises herself as a man and wears yellow-tinted glasses to hide her blue eyes. After losing her biological mother under mysterious circumstances, she was ostracized by the villagers and children, led by Taigen. She eventually went to live with a blind swordsmith, where she learned the art of sword-making and swordsmanship. (Since the blind swordsmith believed in crafting swords tailored to a warrior's style, Mizu had to demonstrate every technique before he would forge one. She learned her skills by observing the swordsmanship of clients who visited and has used them ever since.) Her ultimate goal is to kill all four white men who were in Japan at the time of her birth, whom she suspects could be her father. (One of the four was already killed by Mizu's hand before the start of the story, leaving three remaining.)

A soba chef with a disability, having no hands. He nearly died in a confrontation with a customer carrying a gun at the noodle shop where he worked, but after being saved by Mizu, he accompanies her in hopes of becoming her apprentice. (Mizu naturally refuses and rejects him, even going so far as to leave him buried in a tree or paying a brothel to take him in, but he persistently follows her.) He aids Mizu with his unexpectedly good reflexes. He is the first among the characters to realize that Mizu is a woman.

One of the former bullies who tormented Mizu in the past, he is currently honing his martial arts at Heiji Shindo's dojo. After suffering a humiliating defeat by Mizu during her raid on the dojo—where she cut his hair, leaving the top of his head bald—he pursues her for a rematch. However, through a series of shared events, they somehow form a cooperative relationship. He becomes a sort of frenemy to Mizu, accompanying her through various incidents while waiting for a fair rematch under proper conditions.

The daughter of a noble family and Taigen's fiancée. Serving as the story's femme fatale, she rebels against her family's attempt to break her engagement with Taigen and marry her off to the Shogun. She sets out to find Taigen and becomes entangled with Mizu in the process.

A blind swordsmith. One day, he and Mizu discover a strange metallic object that fell from the sky, leading him to take Mizu in as his apprentice.

The final boss of the story, one of the four white men suspected of being Mizu's father and her main target. An outsider who has amassed wealth and fame by smuggling foreign firearms to the Edo Shogunate, he is of Irish-British descent, as evidenced by his reddish-brown hair and English accent. When he disagrees with the Shogun, he instigates a rebellion and executes him.

A retainer of the Shogun and Fowler's collaborator. He is the only Japanese person connected to Fowler and becomes Mizu's target early in the story.

The madam who runs a brothel in Mihonoseki. She provides Mizu with information regarding Fowler's castle.