Orphaned young after his first adoptive parents were murdered, Kouyuu Li was originally named simply "Kou" by them. He survived stints of child slavery before being abandoned, tied to a sacred tree as a human sacrifice. Rescued by Kou Reishin, he was brought to the capital Kiyou. Following Reishin's marriage to Princess Yuri, Kouyuu became their legally adopted son. Reishin insisted on retaining the surname "Kou" while adding "Li," forming his full name Li Kouyuu. Princess Yuri, his adoptive mother, significantly influenced his development into a responsible person.

Driven by a desire to stand alongside Reishin and repay his kindness, Kouyuu pursued a bureaucratic career. At age 16, he achieved the top rank ("Jougen") in the imperial examinations, becoming the youngest in Saiunkoku's history to do so. This led to his appointment as Vice Minister of the Ministry of Civil Administration, serving directly under his adoptive father, Reishin, the ministry head. Yet despite these accomplishments, Kouyuu faced significant personal challenges, including a notoriously poor sense of direction requiring frequent help navigating even familiar places like the palace. He also exhibited a quick temper and nervousness during public speaking.

Kouyuu's relationship with Reishin was complex; deeply loyal, he often chose to conceal Reishin's neglect of official duties rather than report it, prioritizing family bonds over bureaucratic integrity. This loyalty later resulted in his imprisonment following accusations that he abused his vice-ministerial authority to hide Reishin's dereliction. Though exonerated and reinstated as a general official through Shuurei Kou's efforts, he lost his Vice Minister position. He subsequently rose to Secretary of the Department of State Affairs and ultimately became Prime Minister, later resigning to return to the Kou clan's provincial home.

Beyond family, Kouyuu formed significant relationships. He maintained a close, though frequently denied, friendship with Ran Shuuei, who regularly helped him navigate the palace grounds since their youth. He also tutored Kou Shuurei, developing a friendship with her. The Kou clan leadership, particularly Kou Kurou, proposed a marriage between Kouyuu and Shuurei to secure the clan's future leadership, though this did not occur. Professionally, Kouyuu evolved from seeking Reishin's approval to developing independent convictions. This shift was marked when he accepted Emperor Ryuuki's iris flower, symbolizing his loyalty and belief in Ryuuki's potential as a worthy ruler, defining his own path. In later years, he mentored Hyou Riou Jr., his successor as Prime Minister.

Kouyuu was characterized by bluish-green hair and grey eyes. Despite his professional reputation for diligence and fairness in recognizing others' contributions, personal insecurities and his temper persisted. His legacy includes advancing through high-ranking government positions before retiring to provincial life.