Kirigakure Saizō is a nineteen-year-old ninja from the Iga region and serves as a central figure in the narrative. He is a young man of average height with a lean, well-built physique, black hair, and dark green eyes. His typical attire consists of dark clothing, and he is almost always seen carrying his primary weapon, a large, bandaged kunai called the Mari-Blade, strapped to his back.

Saizō makes his living as an assassin, a profession he approaches with cold and masterful skill. He is known by the fearsome epithet of Marishi-ten, a reference to a deity of light and warfare, which contrasts with his shadowy line of work. On the surface, he appears uncaring, cynical, and aggressive, often displaying a short temper and a sharp tongue. This is particularly evident in his interactions with the Koga ninja Sasuke Sarutobi, whom he constantly calls monkey as an insult, fueled by a deep-seated hatred for the rival ninja clan.

Despite his hardened exterior, Saizō possesses a hidden soft side that he is reluctant to show. This kinder nature emerges gradually, often through small, hesitant actions, such as awkwardly patting the head of someone in distress. His cynical worldview is rooted in a traumatic event from his youth, when he witnessed an elderly samurai, who had served his master loyally, be discarded and thrown into a river after he could no longer fight. This event caused Saizō to deeply distrust the feudal bond between a lord and a vassal, leading him to refuse service to any master and fiercely value his personal freedom. He is a drifter who lives by his own code, choosing not to be attached to anyone to avoid the pain of being thrown away.

Saizō's journey begins when he encounters a young shrine maiden named Isanami. Though he initially tries to leave her, his sense of duty and her persistent nature, coupled with a debt for a bowl of soba noodles, lead him to escort her to Ueda Castle. There, they meet the charismatic lord Sanada Yukimura, who is secretly gathering ten exceptional warriors, each representing an element, to fulfill a greater purpose. Yukimura recognizes the potential in both Saizō and Isanami, identifying Saizō as the Brave of Light. While Saizō finds this ironic given his bloody past, he agrees to stay, driven partly by curiosity about Yukimura's plans and by a growing, protective sense of responsibility for Isanami.

Over time, Isanami's unwavering faith in him and the camaraderie of the other Braves begin to chip away at his emotional walls. He evolves from a solitary, distrustful ronin into a reluctant but committed leader and a dependable ally. While he never loses his edge or his practicality, he becomes noticeably gentler and more accustomed to having people he can rely on, learning to accept the vulnerability that comes with caring for others. His rivalry with Sasuke also transforms, shifting from pure antagonism to a grudging mutual respect and a functional partnership.

As an Iga ninja and a master of his craft, Saizō is a formidable combatant whose abilities are tied to the element of light. His fighting style is fast and lethal, utilizing his large kunai and an arsenal of other tools like hidden blades in his boots, throwing knives, and explosives. One of his signature techniques is Shunko, or Instant Light, which allows him to move with blinding, superhuman speed to slash his opponents in the blink of an eye. Another powerful defensive technique is the Secret Technique, Heaven Returner, where he spins the Mari-Blade to deflect any projectile back toward the attacker. He also possesses more esoteric ninja arts such as Samidare, which involves throwing eight invisible knives, and the ability to create illusory afterimages of himself, likely by bending light. These skills, combined with his sharp instincts and quick reflexes, make him one of the most powerful warriors among Yukimura's ten Braves.