Supporting
Yoo Seung-ho as Kim Chunchu (later King Taejong Muyeol)
Jung Yoon-seok as young Kim Chunchu
Ruler of the next age and ruler of the Three Kingdoms: This series began with Misil's age, continues through Deokman's age and will end at the start of Chunchu's age. This precocious genius will find his own footing alongside Deokman, Yusin, and Bidam, and gain power in his own way. Ultimately he will be the one to uphold the dream that began with the late King Jijeung and unify the Three Kingdoms.
Lee Seung-hyo as Kim Alcheon
Ho Hyo-hoon as young Kim Alcheon
He is most well known as Deokman's staunch supporter and bodyguard. He was initially cold and looked down on Kim Yushin and the Yonghwa Hyangdo. Deokman earns his respect during the war with Baekje, and then his loyalty later on. He is Yushin's war comrade and close friend. Along with Yushin, he is with Deokman until her final days.
Jung Ho-bin as Gukseon Munno
The 8th leader of Hwarang warriors and the Gukseon. Along with Mishil, Seolwon, and Sadaham, they were the people who King Jinheung trusted the most during his era. He helps rescue young Princess Deokman from the palace to protect the royal house. He also took care of Bidam, per the request of King Jinji. He is later killed by Yeomjeong while compiling the Geographical Survey of the Three Kingdoms after Yeomjeong learns that he planned to give the books to Yushin.
Jo Min-ki as King Jinpyeong
Baek Jong-min as young Jinpyeong
Kang San as child Jinpyeong
Deokman and Cheonmyeong's father. He was a weak king. He was placed on the throne after Mishil dethroned King Jinji.
Yoon Yoo-sun as Queen Maya
Park Soo-jin as young Maya
King Jinpyeong's wife, mother of Deokman and Cheonmyeong. She is extremely devoted to her husband. In her early days, Mishil tried to murder Maya after Maya witnessed the Hwarang putting makeup on their faces. Mishil then tried to take Maya's place as queen, however, Munno saved Maya, and her twins, from falling to her death.
Seo Young-hee as Sohwa
Maidservant to Jinpyeong and Maya, foster mother of Deokman. She brought Deokman to the desert and raised her there until Chilsuk found them. She "died" once in the desert trying to save Deokman. She dies a second time in an attempt to protect Deokman also.
Im Ye-jin as Lady Manmyeong
Jinpyeong's older sister; Kim Yushin's mother, and Kim Seohyeon's wife. She had eloped with Kim Seohyeon in her younger days. Her princess status was not restored until the Queen Mother had forgiven her.
Jung Sung-mo as Kim Seo-hyun
Manmyeong's husband; Kim Yushin's father. He is of Gaya descent and on the hwabaek council.
Park Jung-chul as Kim Yong-su
King Jinji's eldest son; Princess Cheonmyeong's husband. He was originally named heir after Princess Cheonmyeong's three younger siblings die (because of the prophecy), and he goes out to war to prove himself. However, he is killed because of Mishil.
In Gyo-jin as Kim Yong-chun
King Jinji's 2nd son, a government minister; Princess Cheonmyeong's ally and confidante. He was a pungwolju (prior to Hojae) and later served on the hwabaek council. He remains loyal to the royal house and becomes Deokman's ally later on.
Shin Goo as Eulje
Senior government minister, friend to King Jinpyeong. He does everything he can to protect the royal house, even if it meant trying to kill Deokman. He is later stripped of his titles by King Jinpyeong.
Jung Woong-in as Misaeng
Mishil's younger brother; the 10th leader of Hwarang warriors. He was also a father to over 100 children.
Dokgo Young-jae as Sejong
Mishil's husband, the Prime Minister and 6th leader of Hwarang warriors
Jeon No-min as Seolwon
Mishil's lover, the Minister of Defense and 7th leader of Hwarang warriors.
Kim Jung-hyun as Hajong
Mishil and Sejong's son, a government minister and the 11th leader of Hwarang warriors
Baek Do-bin as Bojong
Kwak Jung-wook as young Bojong
Mishil and Seolwon's son, a Hwarang commander
Song Ok-sook as Seori
Chief mudang of Shilla, an old friend of Mishil and Misaeng
Ahn Gil-kang as Chilsook
Formerly a Hwarang, agent of Mishil. He was given the task to find the lost twin. 15 years later, he found Deokman in the desert.
Lee Moon-sik as Jookbang
A con artist who rips off the young Deokman and later joins Yu Shin's Hwarang
Ryu Dam as Godo
A con artist who rips off the young Deokman and later joins Yu Shin's Hwarang)
Kang Sung-pil as Santak
Seokpum's, and later Bidam's, aide-de-camp
Joo Sang-wook as Wolya
Last prince of Gaya and the leader of the Bokya. He was adopted by Kim Seohyeon, and then replaced Yushin as Yonghwa Hyangdo's leader. Yushin earns Wolya's loyalty after letting the Gaya refugees stay on the Kim family's private land.
Jung Ho-keun as Seolji
Kayan commander; he is loyal to Wolya.
Choi Won-young as General Gyebaek
Jun Young-bin as Gok Sa-heun
Jung Hyung-min as young Gok Sa-heun
Park Young-seo as Daepung
Lee Suk-min as young Daepung
Go Yoon-hoo as Hojae
The 14th leader of Hwarang warriors (pungwolju), later a council member of the hwabaek.
Hong Kyung-in as Seokpum
Noh Young-hak as young Seokpum
Commander of a Hwarang loyal to Misil. He comes from a poor family, but Misil gives him his elevated status--this is why he is so loyal to Misil.
Kang Ji-hoo as Imjong
Kim Seok as young Imjong
Commander of a Hwarang loyal to Kim Yong-choon
Seo Dong-won as Deokchung
Lee Do-hyun as young Deokchung
Jang Hee-woong as Bakui
Seo Sang-won as young Bakui
Lee Sang-hyun as Piltan
Kim Tae-jin as young Piltan
Kim Dong-hee as Wangyoon
Choi Woo-sung as young Wangyoon
Ryu Sang-wook as Dae Nam-bo
Kim Sang-bin as young Dae Nam-bo
The most prominent of Misaeng's sons, a Hwarang commander
Choi Sung-jo as Seonyeol
Oh Eun-suk as young Seonyeol
Kim Dong-soo as Hyeopseong
Moon Ji-yoon as Siyeol
Shin Tae-hoon as young Siyeol
Jung Hye-sun as Lady Man-ho
Jinpyeong and Manmyeong's mother
Park Eun-bin as Boryang
Bojong's daughter; Kim Chunchu's wife
Qri as Youngmo
Hajong's daughter; Kim Yushin's wife
Mametkulovs Mansur as Katan
Roman, possibly Jewish, trader who teaches Latin to the young Deokman
Seo Kang as Yangkil
O Yeong-su as Wolcheon abbot
Mahbub Alam as Tibetan man (토번인)