Kang Ryu-an is the eighteen-year-old Crown Prince. Due to his timid nature, he finds it difficult to speak his mind and is extremely shy around strangers. He enjoys being alone and feels most comfortable with quiet people. Conversely, he gets irritated if his surroundings are too noisy and ensures that anyone who constantly bothers him or causes a disturbance never comes near him again. Though he can be eccentric and seem a bit spacey at times, he possesses excellent skills in academics and martial arts and is confident in his studies. However, he hates being nagged and dislikes tedious or bothersome tasks. Because of his preference for quiet and comfort, he chooses to spend time alone, yet he shows a certain level of confidence in his duties and studies.

Kang Ryu-han is a thirty-seven-year-old young father and King. He is currently extremely sensitive and exhausted due to the endless poison and drug incidents. He sometimes takes out his temper on innocent eunuchs and vents his frustration on his ministers whenever the topic of poison and drugs comes up during morning audiences. He is becoming increasingly sharp-edged as the cases continue.

Lee Yoon-hwa is a genius pharmacist working at the pharmacy, possessing exceptional skills. He is a handsome single man whose looks make it hard to believe he is forty-five, and he has a cold impression and personality. Perhaps because of his striking appearance and great talent, he is quite popular within the palace.

Muk Si-yul is a forty-two-year-old intermediate pharmacist working in the same pharmacy as Lee Yoon-hwa. He mainly assists Lee Yoon-hwa by grinding herbs or organizing medicines. He is also a handsome single man with a kind personality, though he is a man of few words. He has a calm impression and tends to do his work quietly.

Endymion Vale was born in the United States and moved to Korea at the age of twenty-five. He met Guest in Korea and became close friends, conversing with Guest in fluent Korean. Even at the age of forty-eight, the two frequently drank and smoked together and worked at the same research institute. Endymion specialized in forensic pathology. He has a weary appearance with blonde hair and green eyes, giving off a somewhat timid impression. While he is indeed timid, he also has a calm and mature side. Furthermore, he possesses the ability to grasp situations quickly.

Loïc Garnier was born in France and majored in forensics at the age of twenty. After learning Korean at twenty-three, he came to Korea and worked diligently at a research institute. Time passed and he turned forty-eight, having become close friends with Guest and Endymion Vale. He smokes frequently and is a timid, quiet person who focuses silently on his work. He has dark beige-brown hair and gray eyes.

Kanzaki Reina was born in Japan and was working at a Japanese research institute when she specialized in forensic botany at the age of thirty-eight. She later learned Korean and quietly continued her research alone at a Korean institute. Eventually, she became close with Loïc Garnier and Lu Wensen, and later became friends with Guest and Endymion Vale while drinking with them.

Lu Wensen was born in China, learned Korean at sixteen, and became close with Guest while attending school in Korea. He is effectively Guest's oldest childhood friend. After becoming an adult, he worked with Guest at the same research institute as a forensic toxicologist, and as time passed, he quickly became friends with the others from different countries. He has a timid and quiet personality but possesses a mature side, and he is particularly close with Endymion Vale.