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. His brother Hakuren and their father were killed during this incident, leaving his youngest brother Hakuryuu and his only sister Hakuei the only survivors of Hakutoku's children.
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 Al-Thamen have their way. Hakuryuu wonders if their mother and Hakuei are all right, but his brothers don't answer. After Hakuren dies, Hakuyuu, who is also badly wounded, 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 their mother, and then stabs himself, drenching the little boy in blood to protect him from the flames.
Hakuyuu possessed a great military skills and experiences. He and his brother Hakuren inherited their father's severity and valour making him a potential Great King.
- His name means White Hero.
- "Hakuyuu" is the Japanese pronunciation; in Chinese his name would be Báixióng.