Jinshi is the male protagonist of the story, first introduced as a high-ranking eunuch who manages the administration of the imperial rear palace, where the emperor's concubines and their staff reside. His appearance is strikingly androgynous and beautiful, a combination that captivates nearly everyone who sees him. Despite his official status as a eunuch, this is a carefully constructed facade. In truth, Jinshi is not a eunuch but consumes suppressants to maintain the illusion, a necessary deception to conceal his real identity as a member of the imperial family. His full name is Ka Zuigetsu, and he is the emperor's younger brother, a prince whose lineage was hidden through a child swap at birth to ensure his safety and security within the political labyrinth of the court.

Behind his serene and charismatic public demeanor lies a shrewd and calculating mind. Jinshi is deeply invested in improving the conditions within the palace, using his influence to promote reforms and solve complex problems. However, his personality is far more nuanced than his perfect exterior suggests. He can be deeply jealous, petty, and prone to childish sulking, particularly when it comes to the object of his affection, Maomao. He is highly aware of his own beauty and is not above using it as a tool for manipulation to achieve his goals, a trait that raises questions about his moral compass, though he generally applies it in ways that serve a greater good or to gently steer Maomao toward her passion for pharmacology. His primary motivation shifts from maintaining his hidden status to navigating his burgeoning feelings for Maomao. He is drawn to her precisely because she is the first person he has met who is utterly immune to his charms and judges him solely on his actions. His overarching goal evolves from hiding from his royal responsibilities to accepting them, driven by crises such as the Shi clan rebellion and his desire to protect those he cares about.

Jinshi’s most significant relationship is with Maomao, the young apothecary he plucks from obscurity to serve in the rear palace. He is intrigued by her intelligence and indifference, and over time, his curiosity develops into genuine romantic affection. He persistently seeks her company and grows increasingly possessive and forward in his attempts to secure her place by his side, though Maomao often resists his advances due to her cautious nature and the vast social chasm between them. His loyal attendant, Gaoshun, has been his caretaker since childhood and serves as a constant, grounding presence, advisor, and confidant, often helping to manage Jinshi’s more impulsive or moody tendencies. As the story progresses, Jinshi undergoes significant development. He begins as a man hiding behind a false identity, but after confronting threats like the Shi rebellion and being forced to lead, he abandons his pretense of being a simple eunuch and accepts his duties as the Moon Prince and imperial brother. This acceptance marks a maturation from a sheltered, petulant figure into someone more willing to embrace his power and responsibility in a dangerous world.

Beyond his political acumen, Jinshi possesses several notable abilities. He demonstrates a surprising and substantial knowledge of medicine and first aid, a skill not required for a prince or eunuch. This is evidenced when he carefully carries a severely injured Maomao to minimize jostling to her head and later when he successfully revives her using CPR, indicating deliberate self-study and training in the medical field. He is also a skilled strategist and manipulator of people and political situations, able to orchestrate complex plans and control information flow within the palace to maintain his secrets and further his aims.