Eigetsu To is a 13-year-old male from Seika Village in Koku Province. He has shoulder-length green hair, usually tied in a loose ponytail covering his right eye, and large brown or aubergine-colored eyes. Consuming or smelling alcohol triggers a transformation: his ponytail loosens, his eyes glow red with slitted pupils, and his alternate personality, Yougetsu, emerges. Yougetsu exhibits arrogance, combat skill, and strategic cunning, contrasting sharply with Eigetsu's gentle nature. Born into poverty during civil war, Eigetsu's biological father tried to kill him to save food. Severely wounded, he collapsed in Seika Village and was rescued by physician Ka Shin, who adopted him. The village later fell to a parasitic plague connected to snow foxes, profoundly influencing Eigetsu. At age four, Eigetsu died from his earlier injuries, but Yougetsu—revealed as the White Immortal (Haku Byakuya)—intervened. Sharing half of Ka Shin's life force, Yougetsu granted Eigetsu 20 more years, under the condition Eigetsu's consciousness would fade after that time, leaving Yougetsu in control. Academically gifted, Eigetsu passed provincial exams at age 12 with his village's financial support. Traveling to the capital for imperial exams, he arrived disoriented at a pleasure house, having lost his exam token during Yougetsu's dominance and with no memory of the journey. Despite this, he achieved the top score (Jougen), becoming the youngest in history to do so and receiving a flower bud symbolizing the Emperor's trust. He later served as the youngest-ever co-governor of Sa Province alongside Kou Shuurei. Yougetsu protects Eigetsu during danger, exemplified by securing the exam token with a sake merchant or directing allies to retrieve him. The personalities share a symbiotic relationship, Yougetsu sustaining Eigetsu as the "light" to Eigetsu's "shadow." When Ka Shin fell gravely ill, Eigetsu willingly surrendered 10 years of his remaining life to Yougetsu to extend his adoptive father's life, reducing his own lifespan to one month. During an identical parasitic epidemic in Korin county, Eigetsu confessed his love to Kourin and traveled alone to assist, where Hyou Ren, disguised as Ka Shin, captured him. As his life ended, Yougetsu chose dormancy to resurrect Eigetsu, granting him 50 more years. This act led to Eigetsu's demotion to assistant governor of Sa Province under Kai Yu for recklessness during the crisis. He later volunteered to help Heki Province during a locust plague. Politically, Eigetsu aligns with the Examination Faction, loyal to Emperor Shi Ryuuki. His traumatic past drives his dedication to public service, motivating him to establish medical sanctuaries and prevent knowledge loss from curable diseases.