Kye-Sook (ケイシュク Keishuku) is Soo-Won’s adviser who also acts as his right-hand man.
Kye-Sook is a young man with violet eyes. He has long black hair, with a side fringe covering one of his eyes. He has a small black and gold headpiece on top of his head. He wears a purple overcoat with black hemlines atop a taupe robe held by a black sash. Under this robe is a white innerwear.
Kye-Sook is a cold and serious person who doesn't show remorse to anyone, even to the royalty, if they are to be a liability to his master.  He has a vast knowledge on people and important matters which can make him consider all types of scenarios that may bring calamity to the Kouka Kingdom or to Soo-Won. He can also be an opportunist when a chance arises for their plans, such as when he proposes Soo-Won to be the next king to the generals.
He was seriously injured in a battle once when he was young. He’d become unfit for combat, But was picked by lord Yu-Ho and rose to his present rank. As far as revenge goes, He certainly had a reason to hate king ll.
Kye-Sook joins Soo-Won in visiting the Hiryuu Castle to attend Princess Yona’s 16th birthday. However, he is not seen that much. During the night of the princess’ birthday, he enters the emperor’s room together with some guards to check out on Soo-Won’s progress in killing Emperor Il. To his surprise, he sees Princess Yona hugging the corpse of her father and he quickly understands that she saw everything. He immediately asks Soo-Won to silence her so that there will be no witnesses to their rebellion. Because of that, the princess escapes out of fear, and they chase her outside. Eventually, they manage to pin her down but their success didn't last long because of Hak’s interference who eventually manages to flee with the princess.
Days after the incident, a tribal meeting has been called in the castle. Kye-Sook arrives at the meeting room together with Soo-Won and Han Joo-Doh. In their discussion, the Fire Tribe General, Kang Soo-Jin, points out the need of the new king. Kye-Sook takes this chance and recommends Soo-Won for the throne. Son Mundok, the Wind Tribe General, is against the idea and tries to leave. Kye-Sook tells him that his leaving will only prove of Wind Tribe’s planning of revolt because of the circulating rumors about Hak being a murderer. However, Mundok still leaves despite the warning.
On the eve of the coronation ceremony, Kye-Sook accompanies Kang Tae-Jun to Soo-Won upon his request to see him. As Tae-Jun tells about his endeavor on returning back the princess, they learn of her death and Kye-Sook calls his act as treason. Tae-Jun agrees to him and he asks Soo-Won for punishment but is denied. Tae-Jun pleads on Soo-Won again, but he stops him. The following day, Kye-Sook informs Soo-Won about the acceptance of Mundok to his ascension, as well as the newly appointed general who is younger than Hak. He notices how the Wind Tribe has strong young folks due to their recent generals, and Soo-Won teases him for sounding like an old man. At the ceremony, things are going smoothly until Mundok asks Soo-Won of his plans for the kingdom so that he can tell it to the late Emperor Il. Kye-Sook tries to silence his taunts, but Soo-Won declares his answer to him and to the citizens without any hesitation.
Kye-Sook has great respect for Soo-Won and would do anything to help him. He doesn't let anyone disobey his master and will always give his opinions on the matter beforehand before it's too late. He also acts as Soo-Won's second opinion as he gives another insight on what they should do. However, when Soo-Won already makes a decision, he accepts it and doesn't prolong the matter even if it goes against on what he believes. Even though they have a somewhat serious companionship, they can exchange jokes together occasionally.