Hakuyuu Ren (練 白雄, Ren Hakuyū) was the former first Imperial Prince of the Kou Empire.
He was a tall young man with soft dark blue hair and blue eyes. He had thin, split-ended eyebrows and a mole on his chin, in the same place as his siblings and their mother, Gyokuen. He wore traditional clothes and tied his hair in a bun. Physically, he most resembled his father, Hakutoku.
Hakuyuu had a very severe and serious personality. However he seemed to have loved his brothers greatly, and was not afraid to die for their sake.
Ten years before the start of the story, when his enthronement was imminent, he and his brothers were caught in a large fire during a revolt set up by their own mother. He, his brother Hakuren and their father were killed during this incident.
He is seen in Hakuryuu's memories. He is with his brothers, fighting against Al-Thamen in the palace, which has been set on fire. Hakuyuu says that he can neither die nor let them have their way. Hakuryuu wonders if their mother and Hakuei are alright, but his brothers don't answer. After Hakuren dies, Hakuyuu is also badly wounded, and laments that this is the end for them both before telling Hakuryuu that he must survive and take revenge. He then informs Hakuryuu that the one who wanted to kill them is none other than Gyokuen, and then stabs himself, drenching the little boy in blood to protect him from the flames.
- His name means White Hero.
- "Hakuyuu" is the Japanese pronunciation; in Chinese his name would be Báixióng.