The weapon partner of Black Star, classified as a "shadow weapon" ("dark arm" in the English anime), given that she has the power to transform into a wide variety of ninja weapons, including a kusarigama, a shuriken, a ninjatō, and a smoke bomb. She has a gentle, quiet, friendly, and easygoing personality, and provides a soothing influence among her friends whenever they argue. She is also very patient and tolerant of others, which allows her to put up with Black Star's constant arrogance and showboating, and serves as a highly collaborative partner in battle. Her multiple weapon forms are part of a tradition passed down by her family, descended from a demon weapon created by Arachne. She later acquires her most powerful form, a black katana classified as the "Enchanted Sword," which initially belonged to her corrupted brother Masamune before she defeated him and absorbed his soul. After raiding Baba Yaga's castle, Tsubaki joins the elite meister-weapon unit Spartoí along with Black Star.