* Yi San/King Jeongjo (Lee Seo Jin) - the main protagonist in the series. He is the only son of Crown Prince Sado and Lady Hyegyeong. Extremely kind and generous, he makes frequent trips under disguise outside the palace to see how the common people are doing. The series spans from San's childhood (when he's 11), when Crown Prince Sado is being executed, to about 40 years later, when San himself reigned as king. It was during the beginning of the series where San met Sung Song-yeon and Park Dae-Su. In episode 1, he disguises himself as a junior eunuch and sneaks into the courtyard where his father was imprisoned in a rice chest. With the help of Dae-su and Song-yeon, he manages to see him and obey his father's last wish. The friendship that developed as a result would last the entire series. The plot then quickly fast forwards ten years, when San is an adult and Crown Prince of Korea. While he is still Crown Prince, he falls in love with Song-yeon, and when he becomes king, Song-yeon is made his concubine. San also likes fighting martial arts with the Royal Guards. San dies of tumors at the end of the series.
Yi San (San of the Royal House of Yi), 이 산
Wangsesun Cheoha (His Highness the Crown Prince Grandson), 왕세손 저하
Chusang Cheona (His Majesty the King), 주상 전하
Pseudonym: Yang(Ga) Mu-Duk (Mu-Duk of the Yang Clan), 양(가) 무덕
* Seong Song-Yeon (Han Ji Min) - King Jeongjo's love interest. She is the daughter of a skilled palace artist, and thus she has a fondness for art. Orphaned at a young age, and with a baby brother to take care of, she was taken in by a relative who helped her become a palace maid so that she, too, could gain a standing in the palace. When she was 11, she entered the palace, and her relative sent her baby brother, Seong Song-wook, to the family of a local physician, who was kind enough to adopt him. It was on her first night in the palace that Song-yeon met San, who was trying to get to the rice chest, and Dae-su, who was trying to escape the palace, while she was obtaining food for the older palace maids. Sadly, she and Dae-su were caught and kicked out of the palace. Angry at her, the relative that helped her enter the palace refused to take her in, so Dae-su's uncle, senior eunuch Park Dal-ho, took her in and raised her along with Dae-su. However, enemies of the prince forced them all to flee the Capital. The three fled promising that they would return to the palace to keep their friendship. This they do, and Song-yeon enters the Bureau of Painting, which keeps records of events by painting them, as a Damo (Female Painting Assistant). The promise of returning to the palace is not fulfilled until Song-yeon paints a Kirin for an ambassador from Qing China. Curious to learn more about her, San inquires about her at the Bureau and realized it was Song-yeon. He quickly visits her home where he finds Dae-su and his uncle. From there on out, the three stay together. Song-yeon is portrayed in the series as a beautiful, kind, and intelligent young woman. She manages to rise from a lowly Damo to a real Bureau artist. Her good character and beauty make San fall in love with her, and when he is king, she becomes his concubine. Sadly, few people accept her due to her low status (Damos have peasant status) and the fact that she's an orphan, until she gives birth to a boy when she is made a real concubine. Song-yeon dies of liver cancer while pregnant with a second child near the end of the series, but returns to San one time in a dream just before his death.
Seong(Ga) Song-Yeon (Song-Yeon of the Seong Clan), 성(가) 송연
Seong Sang-Gung (Lady Seong), 성 상궁
So-Yong Seong-ssi (Concubine So-Yong of the Seong Clan), 소용 성씨
Ui-bin Seong-ssi (Royal Noble Consort Ui of the Seong Clan), 의빈 성씨
* Park Dae-su (Lee Jong Soo) - King Jeongjo's favorite military officer. Dae-su was originally a junior eunuch in the palace, but because he wasn't castrated, he wanted to leave, so he tried to flee, when he met Song-yeon and San. Like, Song-yeon, he too got kicked out for helping San. Song-yeon and Dae-su know each other very well, as they practically grew up together. Both of them are orphans, but Dae-su's uncle raised them both. When Song-yeon entered the Bureau, Dae-su took the Military Exam and became a palace guard, and became a guard of the Crown Prince (San). From there, Dae-su was promoted over and over till he became Grand Commander of the Dragon Guard, the finest army in Korea and the king's personal guard. Dae-su started out very dumb and dependent, but when he took the military exam, his tutors helped him become a decent man, intelligent, independent, and calm. Without this change, he might have become a thug. Dae-su is one of the 3 military guards who always follow the king. Song-yeon is Dae-su's love interest, but he gave up trying to court her when she became a concubine.
* Lady Hyegyeong (Kyeon Mi-ri) - Jeongjo's mother. She is the daughter of high-ranking court official Hong Bong-han of the Pyeongsan Hong clan. Ever worrisome over the royal family and the people, she is a model mother of the Korean upper class. Lady Hyegyeong looks down on lower class people and she was the main obstacle that caused Song-yeon suffering as concubine. Yet she was also the first one to acknowledge her as a member of the royal family when she gave birth to a boy. Lady Hyegyeong is also responsible for the multiple concubines brought in to bear an heir.
* Queen Hyoui (Park Eun-hye) - King Jeongjo's primary wife, born into the Kim clan. She was betrothed to San at the age of 10 and married him in her late teens (around 19). Hyoui is a very kind, filial, refined, and honest woman, a model wife of the Korean upper class. She has the respect of almost everyone in the palace, but she does not abuse this power. Unfortunately, the Queen is unable to bear children and thus results in Jeongjo's mother bringing in multiple concubines. Queen Hyoui favors Song-yeon, and it is with her help that Song-yeon became a concubine and gave birth to a son. Queen Hyoui dies soon after King Jeongjo dies.
* King Yeongjo (Lee Soon Jae) - Jeongjo's grandfather. He was the (21st) king of Korea before San took over. Yeongjo has one son, his 2nd son Crown Prince Sado, and a daughter, Princess Hwawan. King Yeongjo is a loving and experienced old man, but he is very firm on San so he can make him the next king. King Yeongjo always suspected his 2nd son of high treason, and throughout the series, that was shown. It was not till just before his death did King Yeongjo discover it had been a plot to frame his son. Yeongjo publicly made it known that it had been a false accusation and had his son's grave repaired. Like San, Yeongjo loved the people and when he knew he would die, that day he sneaked out of the palace and went to his private residence so that he could die among the people, which he did. Yeongjo died in 1777 in his 80s of dementia.
* Queen Jeongsun (Kim Yeo-jin) - King Yeongjo's 2nd wife. She is the daughter of the extremely wealthy and powerful Andong Kim clan. Yeongjo fell in love with her when he was 66 and she was 15. Of all the characters in the series, Queen Jeongsun has the best split character. She is very caring and smart, but deep inside, she has a greed for power, and she holds a silent grudge against San, even though she is his grandmother and she cares about him. Queen Jeongsun is the master-mind behind a plot that was determined to destroy San, a plot that lasted decades.
* Crown Prince Sado (Lee Chang Hun) - Jeongjo's father. He is skilled with the bow, in calligraphy, and in painting. He was accused of treason by his own father, King Yeongjo, and sentenced to death in a rice chest. San came to see him a few nights before he died in secret, and Sado told him to retrieve a painting he had painted of a scene King Yeongjo cherished. Sadly, Sado died before Yeongjo saw the painting. It was Song-yeon who discovered the meaning of the painting and brought it to Yeongjo, 14 years after Sado's death. Yeongjo understood and that was how Sado's name was cleared.
* Princess Hwawan (Seong Hyeon-ah) - Yeongjo's daughter by a concubine. Yeongjo is very fond of her. Hwawan is a very beautiful and filial but impatient young woman. Because she is widowed and has no children, she adopted the orphan Jeong Hu-gyeom, who became a court official. Like the Queen, Princess Hwawan is also a master-mind in the plot to destroy King Jeongjo. This cost her her favor with her father. In the end, Princess Hwawan was stripped of her royalty and titles and exiled to a place far from the capital, where she lived in seclusion under house arrest with only her lady-in-waiting to accompany her.
* Jeong Hu-gyeom (Cho Yeon Woo) - Princess Hwawan's adoptive son. He is incredibly intelligent, and the youngest person ever to be part of the royal court; Jeong is the Royal Secretary. He is very smart and filial to the princess. When he was a child, he was San's study partner at the Confucian Academy. In his teens he studied in Qing China, and as an adult he was very successful. Like his adoptive mother, he was also part of the plot to destroy San. When the participants were punished, Hu-gyeom was exiled and put to death by poison.
* Hong Guk-yeong (Han Sang Jin) - King Jeongjo's right hand man, as well as distant cousin to his mother, Lady Hyegyeong(both are members of the Pyeongsan hong clan). He is a very loyal advisor to Jeongjo. He was also the main tutor who helped Dae-su when he was training for the military exam. Throughout the series, he was promoted to Chief Inspector, Chief of the Royal Guard, and Chief Secretary. His younger sister also became one of Jeongjo's concubines. In the end, he was exiled for trying to kill the Queen.
* Yang Cho-bi (Lee Ip-sae) - a Damo at the Bureau of Painting. At first she was very rude to Song-yeon, but when she fell in love with Dae-su and discovered that Song-yeon was practically his sister, she became super nice to her. At first it was for Dae-su, but eventually she developed a real bond and friendship with Song-yeon. When Song-yeon became concubine to Jeongjo, Cho-bi was selected to be her lady-in-waiting, an attendant court lady of the 6th rank (in Korea, all royalty, palace servants, government employees, and government officials are ranked on a system of 1-9, with 1 being the highest rank and 9 being the lowest rank). After Song-yeon's death, Cho-bi becomes Queen Hyoui's second lady-in-waiting.
* Kim Gwi-joo (Park Young Ji) - Queen Jeongsun's older brother. He is a high-ranking minister of the royal court and holds the title Lord Kim. Very aggressive, he can easily beat up anyone who makes him or his sister look bad. He has a very lively and straight forward personality and is highly loved by her sister. He also is very intelligent and loves travelling to the city of Pyongyang. He plans plots to kill San alongside his sister and Princess Hwawan. After his extreme participation in the plots is discovered, Lord Kim is stripped of his titles and exiled.
* Nam Si-cho (Maeng Sang Hoon) - San's chief eunuch. He follows San everywhere he goes and is very supportive of him.
* Chae Je-gong(Han In Soo) - San's administrative assistant. Chae is also Minister of the Tribunal (in charge of national prisons). When San was a child, Sir Chae was also his tutor. Chae is always of help to San and very supportive of his commands. He is also a trusted adult source of advice to Dae-su, Song-yeon, Hong Guk-yeong, and San.
* Sir Yi Cheon - a Royal Artist at the Bureau of Painting. He is by far the most supportive person of Song-yeon at the Bureau. He is great at painting erotica and drawing sketches but is otherwise a not-so-extraordinary artist. Yi Cheon is very funny and bubbly and is always trying to keep is wife happy so she doesn't kill him.
* Artist Tak - another Royal Artist at the Bureau of Painting. He is by far much more skilled than Yi Chun and also is good friends with Song-yeon. In the last episode, Tak is promoted to a Senior Artist of the 5th rank.