Iemitsu Tokugawa is the central figure of the Ōoku: Tanjō - Arikoto・Iemitsu Hen narrative, secretly serving as the third shogun of the Tokugawa line. Biologically female, she is the illegitimate daughter born after the original male shogun, also named Iemitsu, fathered a child with a common woman during a walk through Edo. After the original shogun died from a plague called the redface pox, which decimated the male population, the powerful wet nurse Kasuga no Tsubone retrieved the infant girl to secretly raise her as the successor, dressing her as a boy and presenting her to the world as the new shogun to maintain the Tokugawa bloodline.

Her childhood was steeped in trauma and manipulation. Forced to live a lie within the confines of Edo Castle's inner palace, she was treated not as a person but as a vessel for producing a male heir. Her grandmother, Kasuga, is said to have ordered the execution of Iemitsu's mother to remove potential rivals, and the young girl was subjected to a severe violation by a bodyguard when she was only thirteen, an assault that resulted in a pregnancy and a child who was stillborn. These experiences left her deeply embittered, emotionally unstable, and utterly distrustful of others. She refused to open her heart to anyone, and her early reign was characterized by a cold, hostile exterior and a profound sense of isolation, feeling she had been reduced to nothing more than a walking womb.

The arrival of the Buddhist monk Madenokoji Arikoto marked a turning point in her life. Forcibly brought into the inner palace by Kasuga to serve as a high-ranking attendant and potential consort, Arikoto was the first person to see past her disguise and recognize her suffering. Initially, Iemitsu was hostile and dismissive, but his unwavering compassion, grace, and genuine desire to heal her emotional wounds gradually broke through her defenses. Their relationship evolved into a profound emotional and intellectual bond. Unlike others who sought to use her for political gain or breeding, Arikoto affirmed her identity and her capability as a ruler, helping to transform her from a bitter, traumatized girl into a more confident and decisive leader.

As shogun, Iemitsu demonstrated sharp political acumen and a shrewd, sometimes ruthless, determination. After the death of Kasuga no Tsubone freed her from the old guard's direct control, she took bold steps to solidify her power and redefine the social order. She publicly acknowledged the death of her father and her own true gender as a female shogun, openly revealing the long-held secret to the feudal lords. By leveraging their own struggles with producing male heirs in the wake of the pox, she was able to secure their acceptance of her rule, setting a critical precedent for future female shoguns. This act was not born of kindness but of a practical understanding that such subterfuge was no longer sustainable. She issued policies that reflected her pragmatic and sometimes harsh worldview, such as ordering the dismissal of most of the men from the inner palace and sending them to the Yoshiwara pleasure quarters.

Iemitsu's primary relationships defined her journey. Her bond with Arikoto was her salvation and her greatest love, though it was a tragic one, as he was sterile and could not provide the heir the nation demanded. Pressured to secure the succession, she conceived three daughters with other male concubines, including Arikoto's disciple, Gyokuei. Her relationship with Kasuga no Tsubone was one of cold manipulation; the wet nurse had raised her for a purpose and supplied her with a succession of handsome men to ensure the bloodline continued, but Iemitsu ultimately outgrew her control. In her final moments, succumbing to illness at the age of twenty-seven, Iemitsu displayed her characteristic mix of authority and lingering vulnerability. On her deathbed, she appointed Arikoto as Senior Chamberlain, tasking him with the care of her eldest daughter and successor, Ietsuna. This act ensured her legacy and bound Arikoto to the inner palace for the remainder of his life, a final, complex gesture of trust and possessive love. Her true history was later recorded in a secret chronicle, ensuring her foundational impact on the Tokugawa shogunate would not be entirely erased.