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Sanjuro is a tall, fairly well-built middle-aged man, often seen wearing traditional Japanese clothing. He wears his long black hair in a ponytail similar to Masamune, which seems to be a trait followed by all members of the Nakatsukasa Clan. Being part of the Nakatsukasa Clan means that he is a Demon Weapon, although his Weapon form is not known.
Although with a rather gruff outlook, Sanjuro has a kind-hearted character and acts like a supportive father figure to the people he cares about, including people both in and out of the Nakatsukasa Clan. Despite the infamy of the Star Clan in Japan, he takes in Black☆Star and treats him like family.He is also calm and wise, shown when he questions Black☆Star's motives for growing stronger, and asking him whether he is prepared to go walk the "Path of the Demon".
Full-Weapon Transformation (全身武器, Zenshin buki): As a Demon Weapon and a member of the Nakatsukasa Clan, Sanjuro is capable of transforming into a Demon Weapon. The weapon form in which he transforms is unknown, though the Nakatsukasa's transformations are classified as Shadow Weapons (Japanese weaponry).
Tsubaki and Black☆Star, in order to try and achieve using the Uncanny Sword, go to the home of the Nakatsukasa Clan in Japan. Sanjuro is immediately there to greet Tsubaki. Being the first time she had come back to her parents' house after her fight against Masamune, Sanjuro comforts Tsubaki, before going on to meet Black☆Star. Despite the boy's eccentricity (which embarrasses Tsubaki), Sanjuro quickly takes to him and welcomes him into the family. Black☆Star is soon back to training, and Sanjuro watches him train, and is impressed by his fighting technique. He asks Black☆Star why he fights, and he replies that he does it to stand out, and that he will beat down anyone stronger than him. Sanjuro asks him whether he will do so, even if that will lead him onto the 'Path of the Demon.' Black☆Star angrily replies that he will continue growing stronger, even if it means walking the Path of the Demon.
- Like Mifune's name, Sanjuro's name is an allusion to actor Toshiro Mifune, known for playing the titular character in Akira Kurosawa's samurai film Sanjuro, a sequel to the popular film Yojimbo, about a crafty but irreverent ronin.