A smart, curious girl, she had to endure hardship when she lost her father and brother at only 11 years old. She believes that the Sword Fraternity is being framed by the government, and she enters the palace as a slave in the Bureau of Music to find the Court Lady who has a butterfly key chain who she believes holds the key to her father's and brother's injustice murder.
Han Hyo Joo as Dong Yi
Due to her diligence solving problems and issues in the palace, she is appointed as a Lady Investigator of the Office of the Investigation. She continues her journey of solving her family's death by searching the palace for the Court Lady who holds that butterfly keychain. Throughout her journey, she finds that the current Queen has been set up by Jang family and is deposed. With the help of her friends, she solves the case one-by-one and brings the Queen back to her throne. The King favours her and falls in love. Dong Yi later becomes Consort Sook and gives birth to the 21th King of Joseon.
Ji Jin Hee as King Sukjong
King Sukjong is the 19th King of Joseon Dynasty. He is a brilliant King, a wise politician and during his reign he has to deal with the clash between the Southern faction and Western faction. With the help of Dong Yi, he is able to prove that all the claims made by the Western faction are indeed lies and he stripes all the ministers from the western faction all their titles and builds a new stable & strong government. A true gentlemen and soft at heart, King Sukjong favours Dong Yi and the Queen.
Bae Soo Bin as Cha Chun Soo
As a member of Sword Fraternity and close friend of Choi's family, Chun Soo promised the Chief to take care of his only daughter, Dong Yi. He enters the palace as a police chief purposely to protect Dong Yi and with Chief Su, Dong Yi and together with the rest of the Royal Guards, Office of Investigations & Police officers, he solves the crimes of Jang's family. He loves Dong Yi but decides that it's better to let her go and asks her to give her heart to the King instead.
Lee So Yeon as Jang Hee Bin
King's mother opposes to her because she was born from a lowborn family. However, she found King Sukjong's favour and enters the palace as his concubine. The Western faction follows her and her brother's plan; she's the mastermind of all the treason and crimes which happens in the palace. She is very ambitious and believes that her power as a Queen will help her to get all her wants. She manipulates the King and hides the Crown Prince's illness from him in order to protect her position as a Queen. She tries to kill Dong Yi several times because she is jealous of her.
Kim Yoo Suk as Jang Hee Jae
Together with her sister, Jang Hee Bin, he masterminds all the crimes. He is cagey and blames other people for his mistakes. All his followers are scared of him and prefer to do everything he asks rather than being killed by him.