Kasuga, also known as Kasuga no Tsubone, was the wet nurse to Tokugawa Iemitsu. Her early life was marked by hardship as the daughter of Saitō Toshimitsu, a retainer of Akechi Mitsuhide. After her father’s execution as a traitor, she endured a vagabond existence before being chosen as Iemitsu’s wet nurse in 1604. She played a critical role in securing Iemitsu’s succession as shogun, though some accounts question the authenticity of these narratives. Kasuga’s influence extended to the establishment and formalization of the Ōoku, the inner chambers of Edo Castle, around 1618. She instituted regulations including a curfew, a ban on male interlopers, and permit requirements for women entering the area. When Iemitsu lacked a male heir, Kasuga took decisive action by introducing concubines to bear his children. These efforts led to the births of future shoguns Ietsuna and Tsunayoshi. Following Iemitsu’s death from the Redface Pox, Kasuga concealed his passing and placed his illegitimate daughter, Chie, on the throne, compelling her to assume Iemitsu’s identity and name. To secure an heir, she forcibly recruited the monk Arikoto into the Ōoku as consort to the female shogun. Kasuga was motivated by a desire to preserve the Tokugawa shogunate and initially aimed to restore patriarchal rule. She viewed the female Iemitsu primarily as a vessel for producing a male heir, referring to her as a "walking womb," which created a complex and often harsh dynamic between them. On her deathbed in 1643, Kasuga ordered the creation of a secret historical record, "The Chronicles of the Dying Day" or "Chronicle of a Dying Day," to document the true events of the female shogunate and the history of the Ōoku. This chronicle later served as a key source for future shoguns like Yoshimune. Her legacy rests on formidable political instincts and her pivotal role in shaping the Ōoku system, which ensured stable succession for the shogunate. However, her actions also sowed the seeds for future political discord among the mothers of Iemitsu’s sons. The shift toward a matriarchal structure resulted from her pragmatic efforts to protect the Tokugawa lineage, despite her original intention to uphold the patriarchy.