Male. Usually called Seol-ssam. He strives to make Ondal, the royal son-in-law, a history genius. He cherishes Robin. He is intelligent and brought Ondal from the future. He is young.

Male. A boy from modern-day South Korea. He likes Pyeonggang. He is poor at studying. The prospective son-in-law of the Goguryeo royal family. Kind-hearted and a glutton. Son-in-law: The husband of the princess (Pyeonggang is the princess).

Female. Princess of Goguryeo. She likes Ondal. She falls in love easily. She is strong and hot-tempered, but sometimes shows maturity.

Male. The third son of the Southern Minister. A Goguryeo noble. Leader of the Taehak Trio, known for his intelligence and judgment. He lacks virtue. A former student of Seol Min-seok. Arrogant and handsome. He treats Ondal poorly when Princess Pyeonggang is not around.

Male. Member of the Taehak Trio. Eldest son of the Eastern Minister. Skilled in speech and writing. Handsome. He treats Ondal poorly when Pyeonggang is not around.

Male. Member of the Taehak Trio. Highly skilled in martial arts. He treats Ondal poorly when Pyeonggang is not around. Handsome.

Male. The student representative of Taehak and a 'monster' who excels in every aspect. Due to his intimidating appearance, large build, and strict personality, the students find him difficult to approach, but he is warmer and kinder than anyone else.