Damdeok

The prince of Goguryeo, and also the reincarnation of the son of Hwanwung. (It should be noted that he looked very similar to Hwanwung in the drama.)
He came across a girl named Kiha while he was reading a book at night, and later came to her again to ask her to heal his father who was poisoned by Hogae's mother.
He had grown to a pleasant young man, but in his heart he always suffered because of Hogae's mother who had committed suicide, causing Hogae to believe that Damdeok killed her. He was later found to be the destined King of Jushin.
In order to protect him before he becomes the king, he asked him to not show his true self to anyone, where in he pretended that he has no skills especially in fighting.
The image he showed to everyone was not hidden just by him and his father but he also trusted Kiha about him being a good fighter.
It also helped him not to be touched by people who wanted the throne, they didn't see him as a threat. As a king, he was despised by the leaders of the four tribes that form his parliament for several reasons. However, he proves himself to be an excellent fighter and a schemer who can win entire castles without a single casualty on both sides. This was done by avoiding killing, and instead providing love for his future people. Despite being betrayed by his officials still he saw them as his people. He was enraged at the idea of the tribal leaders that the king of Jushin must be a powerful warrior who can command his army to dominate the world easily. His intention was to unite the world as a brother, not as a conquerer. At the end of the series, he realizes that the task of the King of Jushin was to answer the gods whether humans needed their help to continue surviving. He answers by unsummoning the heavenly artifacts of heaven to avoid killing more people, especially Kiha who was then about to release her power which would consume the world again (Cheongung/천궁: literally Heavenly Bow) and of the Four Gods. The answer he told the gods was that he that he should return the powers and that humans could carry on by themselves.

Ki-Ha


The mistress of the Dark Phoenix. She was born as a princess in a province of Baekje. On the day of the arrival of the King of Jushin, the forces of Hwacheon struck the province and killed everyone in sight. Before being killed by the warriors, Kiha's mother instructed her to protect her sister, Sujini, no matter what. Kiha obeyed by hiding Sujini in a pot of beans, before fainting in the smoke caused by the fire that the Hwacheon forces started. While unconscious, the heart of the Phoenix exploded in a form of watery energy, extinguishing the fire to save her, thus showing that it choses her as the owner. She was discovered shortly afterwards by Saryang, the master-of-fights of Hwacheon, and taken to the base of Hwacheon clan. The Daejangro put a curse on her to control her, while using her power of the Phoenix to regain his youth. She was trained in martial arts by Saryang, who developed an elder-brotherly feeling for her (although he treated her with high respect to his death).
She met Damdeok by chance one night in the royal library and began to fall for him. Their relationship was going well, until the day that Damdeok's father faked his death as an assassination by Kiha to prevent any harm on Damdeok. She became a cold person since, especially after seeing Damdeok having a good time with Sujini. She came to think herself to have been 'used'. Later finding that she was pregnant, she decided to reform the whole of Goguryeo for her child. She went as far as deciding Hogae to be suitable for the father for her child. At the end of the series, she explodes as the Dark Phoenix, which indirectly helps Damdeok to understand his task as the king of Jushin.

Sujini

The mistress of the True Pheonix. The reincarnation of Se-o. When she was just a little baby, she was hidden in a pot of beans by her sister to avoid her from danger. Then a tribe adorning white cloaks found her.
It was after they saw an explosion, there they found Sujini and saw a heart-black sign on her forehead so they believed that she was the one who could release the black phoenix. The leader was about to kill her, however, one person, Hyeon Go, denied having seen the heart-black sign but a heart-red sign instead. He promised if they proved that she is the one, he will kill her with his own hands.
Sujini was pick up as an orphan by Hyeon Go and considered him as her teacher. Growing up in the tribe where men are her companions she developed a boyish traits, but still she was kept by the tribe tenderly. Sujini’s life was hard so she really "loved" money especially used it for buying wine. The way she earned money was quite interesting and funny. She met Damdeok by chance one day while she was getting in somewhat little trouble and Damdeok helped her. Naturally, they had become friends since then. When she got into trouble again, they were watching a polo games. Therefore one more time Damdeok helped her. At that time she discovered that he is actually a prince.
Damdeok frequently sneaked out of his palace into the streets. Sujini and Damdeok often wandered together and she commonly helped him when he got into trouble. Their relationship was strengthened over time. Sujini also gave a big contribution helping Damdeok to recognize himself as the king of Jushin (because of the conflict with Hogae). She always tagged along with him, even onto the battlefield. Sujini was very good at archery (It should be noticed that the artifact that protected Jushin King's life is also a bow). When she discovered that she was the one that would release the black sparrow she chose to live alone. She thus moved away without Damdeok's knowledge and brought up the son of Damdeok and her sister. After nine years, they met again but Damdeok had by then realized she was the one he truly loved.