This article contains details from the manga that are not covered in the anime. Proceed with caution.
King Hiryuu (緋龍 Hiryū?) is the first King of Kouka. He was the red dragon god who descended from the heavens and took on human form to rule a kingdom on Earth. 2000 years or so after his death, he reincarnated in Yona the current heir to the Kouka throne and his bloodline is still alive and well with Soo-Won being one of his known living descendants.
King Hiryuu is a thin man of average height, whose most identifiable feature is his long, wavy red hair and amethyst eyes.
Hiryuu was a kind and loving dragon god, who helped make Kouka Kingdom a better place. He bore no hatred towards humans even though they turned against him.
Once Hiryuu took on human form, he eventually had to fight against humans. It was believed that humans living in that era were full of hatred and evil - making them forget about their God. The kingdom fell into ruins as a result. King Hiryuu was taken captive by humans who desired power and just as everything seemed to be on the brink of destruction, four dragons came down from the heavens to assist him.
The dragons loved Hiryuu, despised the treatment Hiryuu received and told Hiryuu that they should destroy all the humans as punishment for losing their faith towards the gods and then return to heaven. But Hiryuu rejected them. As Hiryuu had become human, he couldn't help but love the humans for who they are. Even if they hate him or betray him, that's what makes them human. The four dragons understood Hiryuu's decision but couldn't bear to lose him either. Therefore they gave their blood and granted their power to four warriors and told them they were to protect and never betray Hiryuu, and to stay with him to the end.
After Hiryuu along with the first four dragons had cleared the kingdom free of evil, their mission had ended. The red dragon inside of Hiryuu became tired, fell asleep and thus, Hiryuu died. The four dragons mourned Hiryuu's death and then walked separate pathways, leaving the tribes they left behind to develop on their own into the five tribes in the kingdom today.
King Hiryuu was described by Zeno as "just another human", meaning that when he descended from heaven, he lost all his powers. However, the one power that he did hold was his relentless love and forgiveness, as well as understanding. This is what he always showed to humans despite them turning on him. It was something the other dragons could not understand.
King Hiryuu's connection to the original White Dragon was loving, as the former's personality would predict, but at the same time serious and almost standoffish. Gu-En was willing even to die for his master, but then he would turn around and question some of the King's decisions, such as giving Zeno the Dragon Crest.
The Blue Dragon, like all of the others, was entirely devoted to King Hiryuu. Abi seemed to have a more quiet personality, similar to his dragon-descendant Shin-Ah, and it can be seen playing into his relationship with the first Crimson Dragon King. To mention, unlike Shin-ah he does have a sharp tongue.
King Hiryuu had a bond with Shu-ten similar to Gu-En and Abi's - they both cared for and protected each other, but still stood at a distance from each other. Shu-ten's snarky personality clashed with Hiryuu's more gentle and quiet one, so they appeared to be the most distant from each other given the group of dragons.
Zeno was most certainly the one most closely connected with the King. Though Hiryuu definitely questioned Zeno's abilities at first, and wanted to protect him instead of allowing him to fight and risk his life, they soon grew closer together after an encounter at his castle. The Ouryuu showed what powers he had by striking his hand against a stone wall and cutting it badly, but holding it up to show its incredibly fast healing properties. The King, upon watching it heal completely in a few moments, seemed distraught at having so underestimated the Yellow Dragon. He realized that if Zeno could heal so quickly, his power was immortality, and was the ultimate shield for him and many more Hiryuus to come. As a token of his gratitude for protecting him during a recent battle and as something to remember him by for years to come, the King gave Zeno a golden Dragon Crest that he constantly wore around his neck as a reminder of him for years, even centuries to come. Zeno treasures it very dearly to the present day.
Yak-Shi was King Hiryuu's biological son, and succeeded him on the throne. After Hiryuu's death, Yak-Shi became devastated, but quickly took on the throne.
- (To the Dragons) "No, I am human now... even though the humans may hate me, betray me... I cannot help but love them" 
- Hiryuu gifted Zeno the Dragon Crest as a token to remember him.