Susumu Yamazaki operates within the Shinsengumi's covert unit, the Watch, specializing in intelligence gathering and espionage. His duties demand secrecy, utilizing disguises and concealment to observe both enemies and allies. His attire and stealth-based combat often lead others to mistake him for a ninja, a misconception he strategically exploits. Yamazaki exhibits a pragmatic, duty-oriented personality marked by loyalty, bravery, and a strong sense of responsibility. He initially regards Chizuru Yukimura as a potential liability to the Shinsengumi but gradually develops trust upon observing her reliability. This trust fosters collaboration, such as overseeing the compound’s medical supplies and sharing his knowledge of East Asian medicinal practices with her. His family background in herbal remedies and acupuncture informs his secondary role as a medical aide, primarily performing first aid. A significant conflict stems from his position in the Watch, which forbids direct intervention during combat. He struggles with helplessness and self-deprecation when forced to witness allies' deaths without acting. Chizuru helps him reframe his perspective, emphasizing intelligence gathering as vital combat. This renews his pride in his work. He later demonstrates newfound assertiveness by openly challenging Vice-Commander Hijikata’s decisions, advocating for strategic retreats to preserve leadership. During the Shinsengumi’s retreat to Edo, Yamazaki consumes the "Water of Life" (ochimizu) to protect Hijikata, transforming into a Fury—gaining enhanced speed and strength but becoming vulnerable to bloodlust. As a Fury, he fights powerful adversaries like Kazama and Amagiri on near-equal footing. The transformation brings severe psychological tolls, including distorted battle priorities and near-fatal loss of control around human blood. He feels profound shame over his inability to perform medical duties due to the bloodlust. Yamazaki’s relationship with Chizuru deepens significantly. Her choice to stay despite his Fury condition alleviates his fears of isolation, and he openly acknowledges romantic feelings for her. They undertake a mission to thwart her father’s Fury army plans, believing their support roles uniquely position them. After the war, they establish a medical clinic in Edo, studying Western medicine and languages. Yamazaki adopts a quieter, more approachable demeanor, occasionally practicing smiles. They marry, and Chizuru proposes renaming the clinic under his surname. Concerned about his Fury lifespan, they pursue research into a cure together. Historically inspired by a real Shinsengumi officer, this incarnation is significantly younger. He remains the only core member who never wears the group’s signature haori coat during official duties, though later depictions show him wearing it retroactively.