Hyakurin, originally named O-Hyaku or Hayakawa Hyaku, is a female assassin who serves as a lieutenant in the Mugai-ryu, a renegade assassination group. She first appears during the series as a cool, calculating, and deeply tormented beauty. Her past is marked by extreme tragedy and abuse, having been the wife of an abusive samurai named Hayakawa Kakosai Genkei. Her husband was obsessed with achieving fame by training their sickly son, Matsuhiko, to become a strong swordsman. He killed their unnamed twin daughter immediately after birth, finding no worth in raising a girl. When Matsuhiko, who suffered from a hereditary illness, failed to meet his father's brutal expectations, Genkei murdered the eight-year-old boy before Hyakurin's eyes. Driven to madness and despair by the loss of her child, Hyakurin killed her husband in retaliation. This act led to her arrest and a death sentence.

Her execution was halted by Habaki Kagimura, who was impressed by her case and recruited her into the Mugai-ryu, offering her a path to freedom in exchange for her service as an assassin. She is approximately twenty-nine years old during the main events of the story. Her most unusual physical trait is her blonde hair, which she conceals under a black wig. She maintains this color by compulsively bleaching it with strong chemicals, driven by a psychological need to remove the imagined sight of bloodstains from it, a coping mechanism for her overwhelming trauma.

Hyakurin is a woman of sharp intelligence and tactical thinking. Her personality is complex, hiding deep emotional pain behind a facade of control and aloofness. She often displays a blunt, sassy, and abrasive demeanor, using her sharp tongue and commanding presence to keep others at a distance and maintain the upper hand in any situation. She is a master of disguise and an exceptional actress, capable of using flirtation or feigned innocence to lure her targets into a false sense of security before killing them. Despite her cold exterior, she possesses a strong, albeit hidden, sense of integrity and a moral code that prohibits her from killing women or children. She prefers to target the arrogant and the guilty. Her primary motivation throughout much of the story is to earn her freedom from the Mugai-ryu and find some form of peace, though she is constantly haunted by the memories of her murdered son.

Within the Mugai-ryu, Hyakurin frequently operates alongside Shinriji, a fellow member who is deeply devoted to her. She treats him with harsh criticism and physical abuse, a dynamic that masks a more complex and caring connection. She deflects his genuine attempts at a closer relationship, including a marriage proposal, though she later admits to caring for him. Her role in the larger story often involves recruitment and strategic missions. She and her group attempt to persuade the immortal swordsman Manji to join their fight against the powerful Itto-ryu swordsmanship school. She also serves as a pragmatic older sister figure and logistical supporter to Rin Asano, offering her advice and assistance, including helping to plan the infiltration of a fortress.

Hyakurin's journey is one of profound development through immense suffering. In a pivotal mission to assassinate an Itto-ryu member, the plan fails, leading to Shinriji being killed and Hyakurin being captured by enemy swordsmen led by Kinuka. She endures days of brutal torture and repeated rape, during which her right arm is severely smashed and broken, permanently impairing her combat effectiveness. This horrific experience leaves her pregnant. Initially, she has no desire to keep the child, believing it would only serve as a relentless reminder of Shinriji's death and the faces of her rapists. It is the intervention of her comrade Giichi, who offers to be a father figure and frames the child as their last chance at a normal life, that persuades her to continue the pregnancy. Due to her condition and the bargaining of her freedom, facilitated by a deal between Giichi and Habaki Kagimura, Hyakurin is allowed to retire from active duty in the Mugai-ryu. In her final appearances, she is visibly pregnant and takes on a more reflective role, signaling her transition away from a life of violence and towards a tenuous hope for the future.

Hyakurin is not a master swordsman; her skills with a blade are considered mediocre. Instead, her combat abilities rely on intelligence, precision, and unconventional weapons. She has extensive knowledge of poisons, which she skillfully deploys in combat and assassination. Her signature weapon is a collapsible wrist-mounted crossbow named Burafuma, which translates to Little Steps. This device fires poisoned bolts with great accuracy, and when it is collapsed, two blades extend from it, allowing her to stab opponents in close quarters. She is also quick on her feet, highly dexterous, and capable of thinking several steps ahead, even willing to poison herself and take an antidote or line her stomach with sesame oil to survive consuming a toxic substance during a mission.