O-Tsū is a young woman in the story of Samurai Reincarnation whose significance comes from her connection to two central figures: the legendary swordsman Miyamoto Musashi and the exiled swordsmith Muramasa. She is the niece of Musashi's abandoned wife and has been adopted and raised by Muramasa, giving her a personal link to the regrets that define Musashi's presence in the tale.

Her most notable trait is her music. She plays a flute tune that Musashi recognizes, and when he launches an attack in search of Yagyū Jūbei, the sound of her playing is enough to make him stop and withdraw. The melody evidently carries memories of the wife he left behind, and the encounter has a further consequence: in the wake of the attack, Muramasa is moved to finish forging a sword for Jūbei, a blade powerful enough to destroy demonic beings and capable of defeating the resurrected warriors.

O-Tsū's role in the story is both emotional and practical. She serves as a reminder of Musashi's abandoned human ties, and her presence helps set in motion the creation of the weapon Jūbei needs. Near the end of the story, she accompanies Jūbei to the island of Funajima, where he faces Musashi in a final duel and defeats him, making her a witness to the resolution of Musashi's arc. The story does not dwell on her inner thoughts or personal ambitions, and no combat or supernatural abilities are attributed to her; her importance rests on her bonds to Muramasa and Musashi and on the quiet, decisive effect of her music.