The Dark Dragon

A long, long time ago, there were six dragons that ruled heaven: the Crimson Dragon, the White Dragon, the Blue Dragon, the Green Dragon, the Yellow Dragon and the Dark Dragon. The six dragons were great friends and their bond was so strong, it was unshakable.
However, the Crimson Dragon came down to earth and became human. He fell in love with humans and led a kingdom, the Kohka Kingdom. But not long after he became human, he had to fight the humans. The people's hearts filled with evil, they forgot the gods, and the kingdom fell. The Crimson Dragon, too, was seized by power-hungry people. Just before he was about to get killed, five dragons came down from heaven. They begged him to annihilate the humans and come back to heaven. However, he refused and the five dragons were devastated. Not wanting to lose him, the five dragons gave their blood to human warriors to give them power.

These warriors protected the Crimson Dragon until he died. All of them were overwhelmed with sadness, either because they lost someone important or the dragon within them was crying. Soon after the dragon's death, these five warriors thought that the world cannot handle their powers so they went and lived separate lives. No one heard from them ever since.

Until, Princess Yona was born. She ended up finding the four dragons. She thought she found all the dragon warriors because the legend she was told was there were four, not five.

Who is the Dark Dragon warrior? Where are they? What happened to legend? Why didn't they mention the Dark Dragon to Yona?