The King: Eternal Monarch

• Lee Min-ho as Lee Gon / Lee Ji-hun[23]
  • Jung Hyeon-jun as young Lee Gon and young Lee Ji-hun

Lee Gon: The third king of the Kingdom of Corea. He is a mathematician and a rower. He is a compulsive and reticent person. He is sensitive when touched by others and does not easily let go of his skepticism towards someone. He often leaves the palace grounds to wander around, either to tend to his curious mind or to escape the palace life. For him, the Royal Palace represents the safety of home, but it is also his most deadly battleground. When he was eight years old, he witnessed the assassination of his father by his half-uncle, Lee Lim. While he was being choked to death, he was saved by an unknown person with Jeong Tae-eul's identity card.

• Kim Go-eun as Jeong Tae-eul / Luna[24]
  • Kim Si-woo as young Jeong Tae-eul
  • Lee Ye-won as young Luna (Ep. 16)

Jeong Tae-eul: A police officer ranked Inspector (equivalent to Lieutenant in the Korean Armed Forces) in the Violent Crimes Squad Three of the National Police Agency of the Republic of Korea. She has a strong will and concrete personality. As a child, she was interested in crime TV shows which led her to become a detective.

Luna: A criminal in the Kingdom of Corea. Shortly after her birth, she was abandoned in a slum in Busan. With no parents and no identification, no official records document her existence. As she grew older, her arrest record expanded to include burglary, breaking and entering, assault and forgery. Her imprisonment led to her being legally-documented as Luna, a name she gave herself after a stray cat that had survived in the slum. Moreover, she suffers from terminal cancer resulting in organ failure and has only few months left to live.

• Woo Do-hwan as Jo Yeong / Jo Eun-sup[25]
  • Jung Si-yul as young Jo Yeong

Jo Yeong: Captain of the Royal Guard of the Kingdom of Corea, and childhood friend of Lee Gon. He exudes a sharp image and stoic professionalism. Jo Yeong first met Lee Gon at his coronation ceremony, when he was four years old. They share a bond of friendship and brotherhood. Jo Yeong treats Lee Gon as his country and his law, which he must always protect.

Jo Eun-sup: A member of the National Police Agency of the Republic of Korea, and the parallel universe counterpart of Jo Yeong. He is humorous, free-willed and joyous. Currently serving his mandatory military duties, his wish is to get discharged soon. He deeply cares for his younger twin siblings, Eun-bi and Kka-bi, and often helps his parents in looking after them. He also has a crush on his childhood friend, Myeong Na-ri.

• Kim Kyung-nam as Kang Shin-jae / Kang Hyeon-min[26]
  • Moon Woo-jin as young Kang Shin-jae and young Kang Hyeon-min

Kang Shin-jae: A detective in the Violent Crimes Squad Three of the National Police Agency of the Republic of Korea. Although he had an ideal childhood at first, his father was imprisoned for embezzlement and his mother soon fell prey to gambling, leading to a tense relationship between them. He has had a crush on Jung Tae-eul since their high school days.

• Jung Eun-chae as Koo Seo-ryeong / Koo Eun-ah[27]
  • Shin Soo-yeon as young Koo Seo-ryeong

Koo Seo-ryeong: The youngest and first female prime minister of the Kingdom of Corea. Despite being born into a broken family, having a drunkard father and a hardworking mother who runs a fish cake shop, she went on to study hard and became an announcer. Although elegant, confident and independent, she is also extremely ambitious and greedy. She married Chairman Choe of KU Group, before divorcing him for alimony. She became a diplomat and then Prime Minister. Her ambition knowing no bounds, and she next wants to become the Queen of the Kingdom of Corea.

• Lee Jung-jin as Lee Lim / Lee Seong-jae[28]

Lee Lim: Illegitimate brother of King Lee Ho and half-uncle of Lee Gon. He wishes to obtain the Manpasikjeok, a mythical flute which he believes would allow him to attain eternity. He assassinates the former king and takes hold of the flute before young Lee Gon breaks the flute into two halves with the Four Tiger Sword. With one half of the flute, he escapes to the parallel universe after committing treason and kills his counterpart in the alternate reality. He aims to retrieve the second half of the flute, which is now in Lee Gon's possession.

• Kim Young-ok as Noh Ok-nam[29]

Head Court Lady of the Imperial Household

• Jeon Bae-soo as Jeong Do-in[30]

Jeong Tae-eul's father. He owns and runs the Hero Academy for taekwondo as the head instructor.

• Kim Yong-ji as Myeong Na-ri / Myeong Seung-ah

Myeong Na-ri: Jeong Tae-eul's landlady and friend. She runs a coffee shop called The Alley near the taekwondo centre.

Myeong Seung-ah: New staff at the Royal Public Affairs Office.

• Kang Hong-seok as Jang Michael (Jangmi) / Jang Mi-reuk

Jang Michael: New recruit of the Violent Crimes Squad Three

Jang Mi-reuk: Trainee at the Royal Guard.

• Hwang Young-hee as Min Hwa-yeon / Park Sook-jin

Min Hwa-yeon: Kang Shin-jae's mother. After her husband is jailed for embezzlement, she falls prey to gambling. This leads to a tense relationship between her and Shin-jae.

Park Sook-jin: Worker at the Imperial Household.

• Jeon Moo-song as Prince Buyeong, Lee Jong-in

Second-in-line to the throne after Lee Gon. He is a professor and doctor by profession.

• Baek Hyun-joo as Secretary Mo

Lee Gon's secretary

• Lee Hong-nae as Seok Ho-pil

Vice-captain of the Royal Guard

• Park Won-sang as Park Moon-sik

Chief detective of Violent Crimes Squad Three.

• Heo Dong-won as Detective Shim

A detective of Violent Crimes Squad Three

• Ahn Si-ha as Kim Hee-joo
• Song Sang-eun as Kyung-ran

• Kim Bo-min as Yoyo Boy
  • Yoon Jung-hyuk as adult Yoyo Boy

The mysterious boy with a yo-yo who controls the parallel universes (Ep. 3-11, 14, 16)

• Seo Jeong-yeon as Song Jung-hye[31]
  • Ko Eun-min as young Song Jung-hye (Ep. 1-2, 16)

Lee Ji-hun's mother.

• Lee Hae-young as Yoo Kyung-moo

Lee Lim's assistant. He runs the Eosu bookstore in Kingdom of Corea.

• Choi Moo-in as Detective Kang Jun-hyuk from the Kingdom of Corea
• Kang Ki-doong as Secretary Kim, Koo Seo-ryeong's secretary
• Lee Hwang-ui as Koo Seo-ryeong's lobbyist
• Park So-jin as Jo Hae-in, Kang Shin-jae's therapist
• Park Ji-yeon as Park Ji-young, Koo Seo-ryeong's friend
• Tae In-ho as Choe Min-hwan

Chairman of KU Group and Koo Seo-ryeong's ex-husband.

• Kwon Yul as Lee Ho, Lee Gon's father (Ep. 1, 4 & 14)
• Son Kyung-won as Lee Seung-heon, Prince Buyeong's son (Ep. 1, 11 & 15)
• Bae Sung-il as Kim Gi-hwan (Ep. 1, 7-9, 11)
• Park Hoon as Jo Yeong's father (Ep. 1)
• Bae Sung-il as Kim Gi-hwan (Ep. 1, 7-9, 11)
• Kim Ki-moo as a gangster (Ep. 1-2, 6)
• Joo Ye-rim as Min-ji (Ep. 1)
• Jeon No-min as Captain Choi Gi-taek (Ep. 4 and 6)
• Ha Seung-youn as the train booking clerk (Ep. 5)
• Shin Hyun-jong as Lee Jung-hoon, the horse doctor (Ep. 5, 7 & 9)
• Lee Yoon-sang as a government official (Ep. 6)
• Sung Chan-ho as the Deputy Prime Minister (Ep. 6 & 13)
• Park Yong as the military official (Ep. 6)
• Ha Seung-ri as Jang Yeon-ji (Ep. 7-11, 16)
• An Seok-hyun as the ice cream shop boy (Ep. 8)
• Tae In-ho as Chairman Choe Min-heon (Ep. 9-10)
• Bae Bo-ram as a reporter (Ep. 10)
• Min Eung-sik as a government official (Ep. 10 & 12)
• Kim Seung-tae as a government official (Ep. 10 & 12)
• Kang Moon-kyung as a government official (Ep. 10 & 12)
• Park Jung-eon as a restaurant manager (Ep. 12)
• Lee Se-rang as a masseuse (Ep. 13)
• Seo Woo-jin as Secretary Mo's son (Ep. 16)

• Baek Seung-chul as a security guard (Ep. 1)
• Heo Jae-ho as Kim Bok-man, a murder suspect (Ep. 1-2)
• Seol Woo-hyung as Jo Eun-woo / Kka-bi, Eun-sup's younger brother (Ep. 2, 6, 10, 13 & 15)
• Jung Ye-na as Jo Eun-bi, Eun-sup's younger sister (Ep. 2, 6, 10, 13 & 15)
• Yoon Sung-woo as Myung-ho, a physically-disabled boy (Ep. 2)
• Yang Jo-ah as Ms. Baek, Myung-ho's mother (Ep. 2, 12, 14-15)
• Cha Sung-je as the boy at the birthday party (Ep. 2)
• Seo Jang-hyun as the boy at the birthday party (Ep. 2)
• Lee Hye-ra as Park Soo-yeon (Ep. 2)
• Kim Jong-tae as Lee Sang-do (Ep. 2, 7, 9, 11-12)
• Ahn Jung-ho as the arrested criminal mistaken to be a new recruit (Ep. 3)
• Hong Ye-ji as Yoon Bo-young from the police station (Ep. 3)
• Yang Hyun-min as a gambling boss (Ep. 3-4 & 9)
• Lee Ho-cheol as Dalgoo, a gang boss (Ep. 3 & 8)
• Jo Jae-yoon as an arcade worker in charge of the shooting game (Ep. 5 & 7)
• Lee Ji-hyun as Jo Eun-sup's mother (Ep. 10)
• Kim Dong-gyun as Kang Shin-jae's father (Ep. 14)
• Wook Ji-hyun as a rookie in the police station (Ep. 16)

• Kim Wook as Choi Nam-jin / bully

Choi Nam-jin: Lee Gon's colleague (Ep. 1, 4 & 16)

Bully: A high-school bully and Shin-jae's classmate. (Ep. 6 & 16)

• Jang Ha-ran as one of Tae-eul's counterparts named Jeong Hyo-jin's, mother (Ep. 16)

On May 7, 2019, Hwa&Dam Pictures officially announced the production of the series.[32] Along with the series' announcement, it was also confirmed that Kim Eun-sook and Baek Sang-hoon will write and direct the series respectively.[33] Later, it was reported that Jung Ji-hyun will also participate in the direction of the series.[34] On May 8, 2020, while the series was airing, it was revealed that Yoo Je-won will join the production as a director.[35]

The total production cost of the series was reported to be over 30 billion Won (US$25 million),[7][8] which the production company was able to recoup from the broadcast deal with SBS and the overseas licensing agreement with Netflix before the premiere of the first episode.[36][37]

On May 7, 2019, Hwa&Dam Pictures confirmed that Lee Min-ho would star as the male lead,[32] reuniting with writer Kim Eun-sook, with whom he had collaborated previously on SBS hit series, The Heirs (2013).[38] On the 20th, it was announced that Kim Go-eun would play the role of the female lead,[39][40] also reuniting the actress with Kim, with whom she had collaborated previously on tvN hit series, Guardian: The Lonely and Great God (2016).[41] On June 3, Woo Do-hwan confirmed on taking dual roles as Jo Eun-sup and Jo Yeong.[42][43] On June 10, Son Seok-gu and Jung Eun-chae were offered roles in the series,[44][45] with Son deciding not to appear in the series and Jung confirming her role.[46][47] A week later, Kim Kyung-nam confirmed his role.[48]

This series also serves as a reunion for Lee Min-ho and Kim Young-ok, who had a similar on-screen relationship in the KBS hit series, Boys Over Flowers (2009).[49]

The first script reading took place on September 17, 2019,[50] while the Principal photography began on October 23, 2019,[51] and ended on May 28, 2020, after seven months.[52] Lee Min-ho and Kim Go-eun filmed their first scene together at the Gwanghwamun Square, Seoul.[53] The temple scenes, showing Lee Jung-jin painting, were shot at Saseongam Temple located in Mt. Osan, Gurye.[54] The Busan train station which is shown in Kingdom of Corea is not the real Busan Station, instead it was filmed at Yeosu Exposition after the production team changed the labeling of the building.

The rowing competition scene was filmed at Misari Regatta, a boat racing track and park in Hanam City, Gyeonggi Province, about 20 km east of Seoul, which had been constructed for the 1986 Asian Games and the 1988 Summer Olympics.[55] The scene where Lee Min-ho chases after the 'white rabbit' barefoot was filmed at Busan Exhibition and Convention Center (BEXCO).[56] Horse-riding scenes were filmed at Mungwang Reservoir, Goesan and Dadaepo Beach, Busan. Scenes were also shot at Sono Felice Vivaldi Park Equestrian Club.

One of the major filming locations of the series is the bamboo forest, where the door to either of the two worlds is located. The related scenes were shot at Ahopsan Bamboo forest, Busan.[57] Some scenes were filmed at Taepyeong Salt Farm in Jeungdo Island in Sinan.[58] An autumn-themed park scene was filmed at Jungri Sports Park, Daegu.[59] Scenes were also filmed at Woljeonggyo Bridge located in the city of Gyeongju, one of the most popular touristic cities of the country for its cultural heritage.[57] A scene where Lee Min-ho attends a funeral in episode 4 was filmed at Our Lady of Lourdes Cathedral, Daegu, a Catholic Cathedral in Gyesan in the city of Daegu which was built in 1902.[60] Some scenes of the series were shot in Munsan Cathedral Complex in Jinju.[61] Scenes were also filmed at Busan National Gugak Center and Chungmu Traditional Market.[62]

The original soundtrack features popular artists such as Hwasa, Davichi, and Gummy,[63] along with songwriter Gaemi, who produced music for 7 of the singles and penned lyrics for 4 out of the 13 singles featured in the series. The complete soundtrack album was released on June 12, 2020, on the same day as the airing of last episode.[64]

Part 1 of the original soundtrack was released on April 18, 2020 with the single "I Just Want To Stay With You" by Zion.T,[65][66] followed by the single "Orbit" by Mamamoo's Hwasa (Part 2), which was released the following day.[67] At the end of the second week of airing, the singles, "Gravity" by Kim Jong-wan (Part 3) and "Maze" by Yongzoo (Part 4), were released at 6 pm KST, 4 hours prior to the episode's broadcast.[68][69] The following week saw the release of "I Fall In Love" by Ha Sung-woon (Part 5) and "Please Don't Cry" by the duet Davichi (Part 6).[70]

Initially, Part 12 and 13 were supposed to be released on May 29 and 30, but were pushed back because of the postponement of episode 14 from May 29 to May 30.[71]

The series has maintained the No.1 spot on the weekly Wavve drama chart for eight consecutive weeks since its first airing,[13] it also has ranked in the top 10 programs most talked about according to the results of a survey on internet reactions to TV programs in the first half of 2020 conducted by the government agency Korea Communications Commission measuring online posts, online comments, number of views on related video and news articles.[72] Netflix 2020 year-end viewing lists revealed that The King: Eternal Monarch was the most popular Korean drama series in India, Malaysia, Philippines and Singapore and one of the top two in the U.S.[14][15]

On August 6, Studio Dragon announced their financial performance over the second quarter with a record-breaking 135 million dollars in total sales,[73] a 25.9% increase over the same period of time last year,[74] accrediting the boost to license sales of works such as Crash Landing on You, The King: Eternal Monarch, and It's Okay to Not Be Okay,[75] and according to Hancinema, "'The King: Eternal Monarch' likely deserves most of the credit, as it is the only one of these dramas to have aired entirely during the second quarter."[76] Business Korea also reported, "Studio Dragon's TV series sales climbed 9.3% year-over-year on average amid the growing popularity of K-dramas, particularly The King: Eternal Monarch."[77]

The BBQ Chicken brand advertised on the series reported the sale of 550,000 sets of its fried chicken in a month due to its appearance on the series.[78]

The King: Eternal Monarch received mixed reviews.[16][18] The series was highly anticipated by fans before its premiere, as it served to be Lee Min-ho first comeback projected following his release from mandatory military service, and being written by screenwriter Kim Eun-sook.[10]

Arab News praised the series commenting, "In a world of mainstream K-dramas filled with cliches and reoccurring themes, 'The King: Eternal Monarch' stands unique and absolutely mind-blowing,"[80] while Sebastian Roché tweeted, praising the series.[79] The Philippine Cosmopolitan reviewed it saying, "The drama was smart and ambitious. It's commendable that screenwriter Kim Eun-suk aspired to attract a wider audience by choosing to veer away from tried and tested drama tropes."[81] Woo Do-hwan’s performance as double characters was praised throughout the show's run.[82][83][84]

Oppositely, The Korean Herald wrote that the series failed to present adequate explanation of the parallel universe to the audience who complained of the similarity of the two different worlds,[17] with cultural critic Ha Jae-geun describing, "The story unfolds at a relatively slow pace and the parallel world is too complex to understand. It's hard to find some clever, creative conversations and catchy soundtracks in 'The King'."[17] William Schwartz from Hancinema commented on the directing saying, "The lack of coherent direction has also dampened enthusiasm for the drama,” and expressed that, “Viewers have taken issue with the inconsistent intuitive intelligence shown by the main character .... particularly the unapologetic extent to which characters from the fantasy universe play on uncomfortable gender tropes," [85] while Cinemaescapist stating, "Others may find themselves disappointed as the ensemble cast swim in a sea of cliche."[86] The series was also criticized for the excessive use of product placement, with one episode featuring 7 product placement ads,[87] which was seen as undermining the series's content and artistry.[88][89][90][91]

Following its premiere, an online controversy erupted as Korean viewers noticed that the architecture of the fictional world of the Corean Empire resembled that of Japanese temples, in particular, the Japanese temple complex Tōdai-ji and the temple of Kōfuku-ji. As the series was aired during a time when Korean-Japanese relations were uneasy, the production team issued an apology and stated that the buildings in question would be corrected.[92][93][94][95] As the series progressed into its sixth episode, another controversy occurred as the episode depicted a naval war between Japan and the Corean empire, where the viewers noticed that the Japanese warships had the design of present Korean warships.[96] The producer admitted that the production team used open source and stock images as reference material for the Japanese fleet as they were restricted from filming overseas due to COVID-19 and that they should have consulted experts to review the material.[97]

Another controversy also surfaced in the sixth episode about the image of the dragon in the royal robe and the crown of the King. According to the viewers, Emperor Lee Gon's golden crown was similar to crowns used in the Silla Dynasty and his royal robe with the image of the dragon was mainly used in the Joseon Dynasty, and though the series states that it is not set in any particular dynasty in Korea and is a fantasy, they considered this mistake as a massive mix-up.[98][96]

On May 14, the Korea Communications Standards Commission issued an advisory warning against the series for scenes that challenged gender equality after viewers complained that those scenes promote fixed gender stereotypes.[99][100] The scenes in question featured the female prime minister saying, “Bras without wires can’t support the chest,” as well as the scene portraying a boat race where a female spectator said, “Men need to wear less and move around lots.”[101]

On the same day, the Korea Communications Standards Commission also issued a warning to the production when the advertising review subcommittee took issue with the indirect advertising seen in episodes 8, 9, and 12 stating that it violated article 47 of the broadcasting regulations.[102][103]

At the time of airing, the series had the second highest premiere ratings on SBS’s Friday-Saturday drama time slot, proceeded by Nokdu Flower, and the highest in 2020 when it premiered with a TV viewership rating of 11.4%,[12] which went up to its highest recorded rating of 11.6% on the second episode.[105] The rating hit it lowest at 5.2% on the 11th episode,[106][107] hovered around the 6-8% viewership ratings on later episodes,[108][109] and ending with a viewership rating of 8.1%.[110][111] The series saw smaller domestic popularity compared to previous works by writer Kim Eun-sook.[17] According to International Business Times, the development of the plot, the editing and the forced scenes were the reasons the series failed to increase its ratings,[112] while The Korea Times in an interview with an anonymous drama production company official said that the low ratings can largely be attributed to rise of streaming platforms.[113]