Koku-u is a lady-in-waiting in the Jade Pavilion, the palace of the high consort Gyokuyou, in the anime adaptation of The Apothecary Diaries. She is a minor character who serves alongside her two sisters: Haku-u, the eldest, and Seki-u, the youngest. The three sisters entered Gyokuyou’s service shortly before the birth of the Crown Prince. Koku-u is the middle sister and, like the others, assists with cleaning, cooking, and other domestic duties within the pavilion. Her age is nineteen. Her name can be interpreted to mean black feather, which complements the names of her sisters, who are white feather and red feather respectively. In terms of appearance, she has long brown hair tied into a side bun with a black ribbon, and her eyes are purple. Koku-u’s personality and motivations are not deeply explored, as her role in the story is largely that of a background attendant who supports the daily life of the consort’s household. She has no significant character development or notable abilities beyond the ordinary tasks of a lady-in-waiting. Her primary relationships are with her two sisters, with whom she works closely, and with the other ladies of the Jade Pavilion. Her presence contributes to the atmosphere of the rear palace but does not drive the central plot.