Kazuki Muto is a sixteen-year-old boy who attends Ginsei Private Academy alongside his younger sister, Mahiro. He lives in the school dormitory, and his parents are working overseas. As an ordinary high school student, his life changes dramatically when he is killed while attempting to save a girl from a monstrous creature known as a homunculus. The girl, Tokiko Tsumura, is an Alchemist Warrior who was deliberately luring the monster, and she feels responsible for his sacrifice. To revive him, she implants an alchemical device called a Kakugane into his chest, which serves as a replacement for his destroyed heart. This act brings him back to life and thrusts him into the secret world of alchemic warriors who battle homunculi.

Kazuki is defined by his optimistic, enthusiastic, and remarkably kind-hearted nature. He possesses what could be described as a compulsion to help anyone he sees in danger, whether a close friend, a stranger, or even a former enemy who shows signs of being redeemable. His willingness to sacrifice himself for others, which led to his initial death, remains a core character trait throughout his journey. He is also somewhat childish and naive, occasionally displaying a lack of common sense, such as forgetting about an injury or explaining away unusual events with implausible excuses. Despite his serious mission, he retains a lighthearted and energetic demeanor, famously calling out his attack names in battle because he believes it makes him feel more powerful. He has a particular fondness for older women, which is reflected in his taste in magazines, and he has a strong aversion to shiitake mushrooms and mathematics.

His primary motivation stems from a desire to protect others, a drive that is especially powerful concerning his sister Mahiro, whom he is determined to keep safe. After being given a second chance at life, he feels a profound sense of responsibility to use his newfound power to fight against the homunculi that threaten innocent people. He refuses to simply return to a normal life and instead insists on assisting Tokiko in her mission. His goal is not just to destroy monsters but to protect everyone, a principle that often puts him at odds with the more pragmatic and ruthless members of his own organization.

Kazuki’s most significant relationship is with Tokiko Tsumura, the Alchemist Warrior who revived him. Initially, Tokiko is a bitter and serious individual, hardened by a traumatic past, but Kazuki’s persistent warmth, cheerfulness, and optimism gradually chip away at her emotional walls, bringing her out of her shell. Their partnership deepens into a strong romantic bond, though Kazuki often expresses his feelings in awkward or unintentional ways that can fluster her. He also shares a close, almost father-son relationship with his commanding officer, Captain Bravo, who sees a reflection of his own gung-ho attitude in the young warrior. Among his peers, he is popular and well-liked, often hanging out with a group of friends nicknamed the "four fools plus one," which includes his sister.

As the central protagonist, Kazuki serves as the hero who must confront the greatest threats of the story. His role becomes significantly more complicated halfway through the series when it is revealed that the Kakugane serving as his heart is not a standard model but one of the three Black Kakugane. This device transforms its user into a powerful but dangerous being known as a Victor, a type of "walking wasteland" that gradually and involuntarily drains the life energy from everyone around them. Upon this discovery, he is hunted by his former allies, who dub him Victor III, and a special assassination team. Despite this, he is given a six-week ultimatum to find a cure for his transformation before his condition becomes permanent; if he fails, he has promised to destroy himself.

Kazuki’s development throughout the series reflects his struggle with this dark fate. Initially an unskilled but powerful fighter relying on his determination and durable artificial heart, he undergoes intensive training under Captain Bravo to refine his abilities. The greatest challenge to his character is the Victor transformation, which forces him to confront the possibility of becoming the very kind of monster he has sworn to fight. He refuses to accept that he must harm others to survive and actively seeks an alternative, demonstrating his commitment to his ideals even when they seem impossible to uphold. In a final act of heroic sacrifice, he sends himself and the original Victor to the moon to end the threat, though he eventually finds a way to return to Earth and resume a normal life with Tokiko and his friends.

Kazuki possesses the Buso Renkin, or armed alchemy weapon, known as Sunlight Heart. It takes the form of a large lance with a decorative cloth attached. It is said that his personality is suited to a spear because he tends to "poke his nose" into other people's business. The cloth is not merely decorative; it can resonate with Kazuki's willpower and transform into an energy field. He uses this energy for propulsion, launching himself forward in devastating charge attacks, or to cushion high-velocity falls. As the story progresses and his Victor nature awakens, his weapon evolves into Sunlight Heart+, a smaller, sleeker version that lacks the cloth but can release concentrated blasts of internal energy and still generate propulsive force for high-speed charges. Because his Kakugane is his heart, the destruction or prolonged separation of his Buso Renkin from his body would be fatal. His signature techniques include Sunlight Slasher, a quick charged piercing attack; Sunlight Flasher, which emits a blinding light; and Sunlight Crasher, his most powerful move that wraps the lance in energy for maximum destructive force. When he transforms into a Victor, his physical strength, speed, and regenerative abilities are massively amplified, though he cannot control the life-draining effect that comes with this power.