Born Saitō Fuku in 1579, Kasuga no Tsubone was the daughter of Akechi Mitsuhide retainer Saitō Toshimitsu. Following her father's execution after the 1582 Battle of Yamazaki, she endured hardship before being raised by her maternal relative, aristocrat Sanjonishi Kinkuni. Under his tutelage, she received an elite education in aristocratic arts including calligraphy, waka poetry, and incense mixing. She married Kobayakawa Hideaki retainer Inaba Masanari, bearing three sons—including Inaba Masakatsu—and adopting Hotta Masatoshi. Her pivotal role came during the 1600 Battle of Sekigahara, where she convinced Kobayakawa Hideaki to defect to Tokugawa Ieyasu's Eastern Army, securing its victory. This action led to her 1604 appointment as wet nurse to Tokugawa Iemitsu (then Takechiyo), heir of Shogun Tokugawa Hidetada. Her aristocratic background, prior motherhood, and husband's military service contributed to this selection. She developed a deeply attached relationship with Iemitsu, causing tension with his biological mother, Oeyo no Kata. Within the Ōoku: The Inner Chambers alternate history, a fictional "redface pox" pandemic decimates Japan's male population, killing the male Iemitsu. Kasuga orchestrated a cover-up to preserve the Tokugawa line, forcing Iemitsu's illegitimate daughter, Chie, to assume his identity as shogun. To maintain this deception, Kasuga transformed the Ōoku—originally Edo Castle's women's quarters—into a structured male harem. Young men were recruited as concubines to produce heirs. She specifically kidnapped noble-born monk Arikoto, compelling him to become Chie's consort under the name "Oman" within the Ōoku hierarchy. Kasuga wielded unprecedented authority as the Ōoku's de facto ruler, formalizing it with strict protocols: curfews, restricted male access, and mandatory resident permits. She documented its operations in the confidential "Chronicle of the Dying Day," a record later vital to shogunate archives. Her political reach extended beyond the Ōoku; she negotiated with the Imperial Court during a 1629 diplomatic crisis. To gain legitimacy for an audience with Emperor Go-Mizunoo and Empress Meishō, she arranged adoption into the aristocratic Sanjonishi family, earning the court title "Kasuga no Tsubone" and later Junior Second Rank. Her actions ensured Tokugawa continuity through matrilineal succession but imposed severe personal costs: Chie endured isolation and erased identity as the false Iemitsu, while Arikoto and others suffered lost autonomy. Kasuga justified these sacrifices as necessary for stability amid pandemic-induced societal collapse. She died in 1643 at age 64, reportedly refusing medical treatment during an illness in symbolic loyalty to Iemitsu, who had survived smallpox. Her final poem expressed Buddhist themes of release from worldly suffering. Her legacy persisted through the enduring Ōoku system, central to shogunal succession for generations. The Chronicle of the Dying Day preserved her account of the redface pox and the shogunate's matriarchal transition, later discovered by the eighth shogun Yoshimune. This record revealed Kasuga's foundational role in reshaping Japanese society, alongside the controversial, ruthless pragmatism of her methods.