Haku-u is a lady-in-waiting serving at the Jade Pavilion, attending to the high-ranking consort Gyokuyou. She is the eldest of three sisters who are nearly identical in appearance, and she distinguishes herself by wearing a white ribbon in her hair, a choice that aligns with the literal translation of her name, which means white feather. Her younger sisters are Koku-u, who wears a black ribbon, and Seki-u, who wears a red one. Originally from the Western Capital, she is a member of the Yi Clan and grew up as a childhood friend of Gyokuyou. Haku-u shares an age with Gyokuyou and comes from the same hometown. She was five years old when a rebellion occurred against the Yi clan, an event that forced her, her mother, sisters, and a young Rikuson to evacuate and hide in the home of Gyokuyou's mother to escape from supporters of Gyoku-ou. She later became one of the new ladies-in-waiting for Gyokuyou in the rear palace, accompanied by her sisters.

Personality wise, Haku-u is protective of Gyokuyou and deeply cares for her well-being. She exhibits a mature and serious demeanor, being described as the most mature of Gyokuyou's ladies-in-waiting aside from Hongniang. This seriousness makes her distrustful of anyone she believes might take advantage of Gyokuyou, including her relatives Gyoku-ou and Gyokuen. She is not afraid to voice her criticisms either, as she once chided Gyokuyou for a carefree attitude when a problem arose with a blemished hair stick. Others have noted her sharp intellect, with the character Rikuson recalling that she was already very smart at the age of five.

Within the story, Haku-u's primary role is as a supporting attendant to Consort Gyokuyou, especially during the period following the birth of the Crown Prince. She takes her duties diligently, and on one occasion, she took care of the protagonist Maomao when the young woman spent a night at the Jade Pavilion. Her key relationships are firmly rooted in her past. Her bond with Gyokuyou is long-standing, originating from their shared childhood in the Western Capital. Similarly, her connection to Rikuson is significant, as they were both evacuated together as children during the rebellion. Years later, Haku-u would urge Rikuson to become the leader of the Western Capital when he returned there, and she took on the responsibility of sending Gyokuyou's letters to him by carrier pigeon, indicating a strategic and trustworthy partnership.

Haku-u shows notable development in her transition from a childhood survivor of a political uprising to a trusted and proactive lady-in-waiting. Her actions demonstrate a commitment not just to Gyokuyou's personal safety, but also to her political interests, as seen in her secret correspondence with Rikuson. Her notable abilities include her diligence in her duties and her capacity for careful, secret communication, which proves useful for maintaining contact with allies outside the palace.