Yunoshin Mizuno is a young samurai who becomes a significant figure within the Ōoku, the inner chambers of Edo Castle, during the reign of the eighth female shogun, Tokugawa Yoshimune. He is the son of a hatamoto, a high-ranking samurai family that has fallen on hard times. From a young age, he is in love with Nobu, the daughter of a wealthy merchant, but their difference in social standing makes marriage impossible. To secure financial support for his family and allow his sister to afford a husband, Mizuno decides to enter the Ōoku as a page, despite the understanding that he can never leave or speak of what happens inside. His motivation is primarily economic duty and a desire to spare his beloved the shame of a poor match, as he believes his departure will allow her to accept a more suitable suitor.

Physically capable and quick to learn, Mizuno adapts to the strict and often corrupt world of the Ōoku under the guidance of an experienced steward named Sugishita. He becomes increasingly aware of the injustices and wastefulness of the institution, which houses thousands of healthy young men as a display of the shogun’s power while the country suffers from a skewed population caused by the redface pox. His straightforward nature and samurai discipline draw the attention of the Ōoku’s chief chamberlain, Fujinami, who promotes him from a menial servant to the rank of groom of the bedchamber, positioning him to become a concubine. When Yoshimune becomes shogun, she notices Mizuno and selects him as her first consort. Unbeknownst to Mizuno, an old custom decrees that the first man to sleep with a virgin shogun must be secretly executed afterward, a fate Fujinami intended for him to spare his own favorite. Yoshimune, however, refuses to follow this tradition and instead devises a plan to spare Mizuno’s life. She gives him a new identity and allows him to marry Nobu, permitting him to leave the Ōoku and live in secret.

Mizuno’s personality is marked by a sense of duty, resilience, and a quiet emotional depth. He is not ambitious for power but rather pragmatic, accepting his position to fulfill family obligations. He shows empathy toward others in the Ōoku and is disturbed by the moral decay he witnesses, yet he maintains his composure. His relationship with Nobu is the emotional core of his arc, representing the personal sacrifice demanded by the society’s gender roles. His interactions with Yoshimune are more formal and transactional, though she comes to respect his integrity. His development from an impoverished samurai to a concubine and then to a freed man highlights the arbitrary cruelty and occasional mercy of the system. Notable abilities include his swordsmanship, his capacity to observe and navigate court politics without drawing undue hostility, and his physical appeal, which initially attracts the shogun’s favor. Mizuno’s story serves as the audience’s entry point into the world of the Ōoku, introducing the setting’s rules and tensions before the narrative shifts to the earlier historical flashback of the third shogun’s era.