Eigetsu To is a major supporting character in the second series of The Story of Saiunkoku. He is a young prodigy who becomes one of the youngest government officials in the history of the fictional empire of Saiunkoku. His background is marked by profound tragedy and a supernatural pact that dictates his very existence. Born into extreme poverty during a time of civil war, his biological family attempted to kill him at the age of four to conserve food for his siblings. Left with fatal wounds, he was discovered near the village of Seika by a traveling physician named Ka Shin, who saved his life. However, Eigetsu had already died from his injuries; he survives only through a contract with a supernatural entity, later revealed to be the White Immortal Byakuya, who resides within him under the name Yougetsu. This contract granted him a limited lifespan of approximately twenty years. He was then raised by Ka Shin, who adopted him and taught him medicine, shaping his compassionate worldview.

Eigetsu's personality is generally gentle, studious, and deeply compassionate. He is a diligent and hardworking individual who possesses an unassuming nature, often masking his immense intelligence. This primary personality is in stark contrast to his hidden persona, Yougetsu. When Eigetsu consumes or even smells alcohol, Yougetsu emerges. This secondary personality is aggressive, strategic, arrogant, and possesses worldly knowledge and martial skills that Eigetsu lacks. Yougetsu acts as a protector but openly disparages Eigetsu, creating a complex and often contentious symbiotic relationship between the two. Eigetsu himself resents Yougetsu's emergence, as it not only causes him to lose control but also actively drains his already shortened lifespan.

Eigetsu's primary motivation is to fulfill a promise made to his adoptive father, Ka Shin. Having witnessed the devastating plague that destroyed Seika village, he is driven to establish a medical haven where knowledge can be gathered and preserved to cure any illness. He abandoned his original dream of becoming a doctor and instead pursued a career as a government official, believing this would give him the power and resources to create such an institution. This goal is rooted in preventing the preventable deaths he witnessed in his youth.

In the second series, Eigetsu's role in the story is significantly expanded. Having achieved the top score on the imperial exams at the age of thirteen, making him the youngest jougen in history, he is appointed as the co-governor of Sa Province alongside the protagonist, Shuurei Kou. This appointment is historic, as they become the empire's first co-governors. Together, they work to restore order to the province following a rebellion and to establish the academy Shuurei envisions. His work in the province leads him to confront a resurgence of the same plague that destroyed his home village, an event that forces him to confront his own mortality and the terms of his contract with Yougetsu.

His key relationships are central to his character development. He shares a profound bond of friendship and mutual respect with Shuurei Kou, whom he considers a close friend and ally. He also forms a significant relationship with Kourin, a young woman who initially serves as Shuurei's decoy. He develops deep feelings for her, eventually confessing his love and marrying her later in the story. His relationship with the entity Yougetsu is arguably the most defining of his life, a constant reminder of his borrowed time and a source of both power and frustration. He also shares a special friendship with the eccentric nobleman Ryuuren Ran, who considers him one of his "heart's friends".

Eigetsu undergoes significant development, centered almost entirely on his relationship with his own mortality. He begins as a gifted but fragile boy living on borrowed time. Over the course of the series, he makes critical decisions that further reduce his lifespan, such as transferring half of his remaining years to save Ka Shin from the Seika plague. When he has only one month left to live, he bravely sets out alone to assist villages afflicted by an epidemic, accepting his fate. At the moment of his death, Yougetsu intervenes, choosing to go dormant within Eigetsu's body. This act extends Eigetsu's life by another fifty years, effectively allowing him to survive and removing the periodic threat of Yougetsu's emergence, as the entity falls into a deep sleep. This transformation allows Eigetsu to move forward with his life, marrying Kourin and continuing his work.

Eigetsu's notable abilities are diverse. He is an academic prodigy with a remarkable intellect for law, medicine, and governance. He is a skilled physician, having been trained by his adoptive father from a young age. He also possesses specialized knowledge in epidemiology, which he uses to diagnose and combat the plague that reappears in Sa Province. When his Yougetsu personality emerges, he gains temporary access to extraordinary abilities, including enhanced martial arts prowess, high tolerance for alcohol, and a shrewd, tactical mind. However, the use of this power comes at the direct cost of his own life force.