Eigetsu To possesses shoulder-length green hair, typically tied in a loose ponytail, large brown eyes, and a fringe covering his right eye. His appearance shifts dramatically when his alternate persona Yougetsu emerges: his hair loosens and his eyes glow red with slitted pupils. The associated White Immortal form features white hair and cat-like green eyes.

Born into extreme poverty during civil war, Eigetsu's biological family attempted to kill him at age four to conserve food for other children. Suffering fatal injuries, he survived through a supernatural contract with the entity Yougetsu—later revealed as the White Immortal Byakuya—granting him a limited lifespan of approximately 20 years. Physician Ka Shin rescued him near Seika village and adopted him, raising Eigetsu as his successor. When a plague caused by parasitic contamination from snow fox feces later destroyed Seika village, Eigetsu transferred half his remaining lifespan to save Ka Shin, drastically reducing his own life expectancy.

Eigetsu harbors a dual consciousness. His primary personality is gentle, studious, and compassionate. Exposure to alcohol, however, triggers a transformation into the aggressive, strategically brilliant Yougetsu. This secondary persona displays distinct martial prowess, alcohol tolerance, and worldly knowledge. Their relationship is symbiotic yet contentious; Yougetsu protects Eigetsu while disparaging him, and Eigetsu resents the persona's emergence. The contract dictates Eigetsu's consciousness would dissipate upon death, leaving Yougetsu in control.

Professionally, Eigetsu became the youngest candidate in history to pass provincial exams at age 12. Funded by his community, he traveled to the capital for imperial exams. Yougetsu's interference caused him to lose his exam token and arrive disoriented at a pleasure house. Despite this, he achieved the top score (Jougen) at age 13, surpassing previous records. His academic success prompted numerous marriage proposals, all of which he refused, resulting in workplace harassment.

Appointed co-governor of Sa Province alongside Shuurei Hong, Eigetsu became the youngest person to hold the position. While protecting Kourin, who was impersonating Shuurei, he was captured by rebels (Satsujinzoku). Rescued by allies including Yougetsu, he later confronted an epidemic in Korin county identical to the Seika village plague. Recognizing his imminent death—now within one month due to prior lifespan transfers—he confessed his love to Kourin and traveled alone to assist affected villages. There, he discovered water purification techniques withheld by a manipulative cult (Jasenkyou) and was captured after encountering Hyou Ren disguised as Ka Shin.

At death, Yougetsu intervened by entering dormancy within Eigetsu's body, extending his lifespan by 50 years. This allowed Eigetsu to survive but resulted in professional demotion due to perceived recklessness during the epidemic. He subsequently served as assistant to his former guardian Kai Yu, the new Sa Province governor. When locusts plagued Heki Province, he volunteered with a medical team, leveraging his epidemiological expertise.

Eigetsu eventually married Kourin, whom he met during their Sa Province governance. His friendships with Shuurei Hong and others provided emotional stability amid societal preference for Yougetsu. His lifelong dedication to establishing medical havens stemmed from witnessing preventable deaths in Seika village and his adoptive father Ka Shin's ideals.