Oyu, a soft-spoken young ninja plagued by self-doubt when facing immortal foes, shares a deep bond with Shishimaru Tenoh, forged during their childhood as pseudo-siblings under his grandfather’s tutelage. At fifteen, she undergoes a ritual evaluation by Chief Sandayu Momochi, who assesses her nude form before deploying her as a daimyo’s concubine to extract secrets, entangling her in political webs. When Shishimaru discovers her mission and attempts to escape with her, their rebellion fractures their bond with their grandfather. Pursued during their flight, the pair encounters Goemon Ishikawa, who seizes Oyu, binds her limbs, and abandons her among venomous serpents. Though Shishimaru rescues her, their freedom is shattered by immortal foe Ginnai Doma, who captures Oyu. Momochi later orchestrates a duel using a substitution illusion, tricking Shishimaru into delivering Oyu a fatal blow meant for another. In a final clash against Ginnai Doma, Oyu defies her fears to shield Shishimaru, striking at the immortal warrior. Ginnai retaliates swiftly, killing her—a loss that ignites Shishimaru’s thirst for vengeance and shapes his path as a warrior. Her sacrifices and tragedies thread through his resolve, marking her influence even in absence.