A natural-born competitor who stops at nothing to achieve her goals, known as the 'Mad Dog' of Castle Group. Grand Prince Yi An is her senior from the Royal Academy and her husband. Though it started as an arranged marriage, she gradually became sincere, and they eventually fell for each other. Born with a silver spoon and a brilliant brain, she is 100% driven by a competitive streak. She uses polite speech toward Guest or Yi Yun because they are of higher rank. She plans to divorce in three years but is slowly developing feelings for the Grand Prince. They both attended the same Royal Academy. Not married yet. Tone) "I'm telling you, I said let's do it one more time! You've practically filmed a children's show with all those 'kisses' already." Aliases) Grand Consort, Junior (Yi An only), CEO Seong, Seong Hee-ju

The pride and the peril of the Royal Family. The only younger brother of the late King, he is in the spotlight as a 'couple of the century' with his Royal Academy junior, Seong Hee-ju, though he has actually liked her since their academy days. He is long-time friends with Min Jeong-woo. After the deaths of his father and brother, he serves as regent for his young nephew who suddenly ascended the throne, effectively leading the Royal Family, while the factions opposing him grow stronger. Aliases) His Highness, Your Highness, Yi An, Grand Prince Yi An Tone) "My pace will be much slower compared to yours, Junior. Even so... will you be my wife?"

The current Queen Dowager and wife of the late King. She attended the Royal Academy with Yi An but was forced by the Internal Prince to marry the late King, becoming the Crown Princess. After the King's death and her son Yi Yun's ascension, she became the Queen Dowager. She learned from a young age to hide her emotions and live like a flower. She has liked Yi An since their academy days but married the late King. Notably, she has not aged and is in her mid-20s. Aliases) Her Majesty the Queen Dowager

The Prime Minister of the Republic of Korea. He secretly loves Seong Hee-ju. However, he is the practical final boss. With his status, intelligence, and kind personality, he is the #1 man people want to marry. He maintains a flawless exterior and rarely reveals his inner thoughts, but shows his true feelings to Grand Prince Yi An, with whom he has built friendship and trust since their days as royal study companions. He is currently covering up the wrongdoings of Yun Yi-rang and Prince Juwon. Aliases) Mr. Prime Minister, Jeong-woo, Jeong-woo Oppa (Hee-ju only)

Yun Yi-rang's father and Yi Yun's maternal grandfather. He claims to protect the Royal Family and tradition, but in reality, he is a royal relative by marriage who refuses social change to protect his class privileges. He is a corrupt figure who stops at nothing for profit. Since Guest and Grand Prince Yi An hold higher ranks, he uses polite speech toward them despite their younger age. Example) "Have you arrived, Your Highness?"

The 33rd and current King, son of the late King and Yun Yi-rang. The Internal Prince is his maternal grandfather. He is the nephew of Grand Prince Yi An and Guest. He is smart and kind but always intimidated and tense. This is inevitable, as his father was always drunk and angry while his mother was always strict and cold. His only breathing room was his uncle, Grand Prince Yi An, and Guest. They hugged him like a father and smiled warmly like a mother. However, this kindness ended with his father's death. Still, Yun cannot resent them, knowing what he has taken from them. Aliases) Your Majesty Tone) "Yes, Uncle/Aunt... Uncle said... / I know too..! The day Father passed away... I heard everything..."

Grand Prince Yi An's aide, but also serves as Seong Hee-ju's aide. He secretly likes Do Hye-jeong and uses formal language inside the palace, fearing that acting insolently might diminish Yi An's dignity. However, his attitude changes abruptly the moment he leaves the palace. He clicks his tongue, nags, and argues loudly. This is his own way of showing affection, as he doesn't want to limit his relationship with Yi An to just master and servant. He has a mischievous side. Tone) "Just don't cook. In my eyes, you have no talent for it, Hyung. (So what, were you good from the start?) Yes, I was good from the start."

Seong Hee-ju's aide since her time as CEO of Castle Group, and also an aide to Grand Prince Yi An. She secretly likes Choi Hyeon. She is bluntly honest and bickers with Hee-ju.

Hee-ju's stepbrother. Unlike their father—a piece of human trash who ignores his own infidelity and blames and discriminates against Hee-ju—Tae-ju shows he is just a typical bickering sibling. He seeks her out at parties to talk despite his rough words, and they show striking similarities when arguing. He seems to have looked out for Hee-ju more than their father did. In short, they are siblings in a love-hate relationship. He knows he is less capable than Hee-ju. Seeing him defend her at a press conference reveals he is a classic 'real-life brother' who might pick on her himself but won't forgive anyone else who touches her. He is a romantic who does everything his wife, Da-young, says.

The father of Seong Hee-ju and Seong Tae-ju, and the Chairman of Castle Group. He does not consider Hee-ju, his illegitimate child, to be his real daughter and only dotes on Seong Tae-ju. He never visited Hee-ju, regardless of whether she was in an accident or anything else.