Main •
Yum Jung-ah as Han Seo-jin / Kwak Mi-hyang : An overzealous mother who wants nothing more than for her daughter to get into Seoul National University's medical school. She hides her past from everyone except her husband and in-laws who know that she grew up poor with an alcoholic father who
sold oxblood and offal. She is the mother of Kang Ye-seo and Kang Ye-bin; the wife of Kang Joon-sang and the daughter-in-law of Madame Yoon. •
Lee Tae-ran as Lee Soo-im : She is a well-known tutor only accessible to the elite and only caters to two students at a time. Because of her 100% success rate in getting students accepted, Seo-jin was determined to hire her as her daughter's tutor. She tutored Young-jae and Ye-seo. She hides a past that she tries to escape from. She has a daughter, Kay (or Katherine), who was an acclaimed genius when they lived in Fairfax.
Supporting Kang family •
Jung Joon-ho as Kang Joon-sang : Seo-jin's husband. A well respected orthopedic surgeon at Joonam University Hospital,who is later promoted as the chief physician at the Spine & Joints Centre. He holds Min-hyuk in absolute contempt and delights in the latter's misfortunes. He is obsessed with maintaining a good track record. He appears to be moral and put off by dishonest behaviour but is actually ambitious and wants to climb the social ladder. •
Kim Hye-yoon as Kang Ye-seo : Seo-jin's elder daughter. Her dream is to enter
Seoul National University's medical faculty and be a third generation doctor. She is studious, determined, and often aces her exams, but is pampered, and wilful. She has feelings for Woo-joo. •
Lee Ji-won as Kang Ye-bin : Seo-jin's younger daughter. She is cynical and often clashes with her sister and believes her parents only care for Ye-seo. She, however, gets along well with the other children in the complex and is the first one in the family to warm up to Hye-na. •
Jung Ae-ri as Madame Yoon : Joon-sang's mother. She is the one who pushed Seo-jin to bring Ye-seo to the top in order to have a third generation doctor in the family.
Hwang family •
Choi Won-young as Hwang Chi-young : Seung-hye's daughter. She is a marketing director of a nightclub. She lied about being a Harvard University student and got caught by her parents. •
Kim Dong-hee as Cha Seo-joon : Seung-hye's elder son; a twin of Ki-joon. A classmate of Woo-joo. Even though he is calmer than his younger twin, he strongly dislikes his father's method of teaching and studying. •
Jo Byeong-kyu as Cha Ki-joon : Seung-hye's younger son; a twin of Seo-joon. He likes to play tricks on Ye-seo. He strongly dislikes his father's method of teaching and studying. •
Kim Joo-ryoung as Noh Seon-hye : Seung-hye's elder sister. She lives in
New York, taking care of her niece Se-ri who moved to the United States.
Woo family •
Jo Jae-yoon as Woo Yang-woo : Joo-young's secretary. • Jo Mi-nyeo as Kay / Katherine : Kim Joo-young's daughter. As a result of a car accident that caused her serious brain damage, she became mentally ill. Implied to have
savant syndrome.
Others •
Kim Bo-ra as Kim Hye-na : Sinah Secondary School student. Later is revealed that she's the daughter of Kang Joon-sang. She is treated as an enemy by Ye-seo and is also a tragic character who needs to survive in a cruel and selfish world. She has no feelings toward Hwang Woo-joo. •
Woo Ji-hyun as Jeon Jin-man : Neurosurgery resident • Song Min-hyung as Choi In-ho : Medical superintendent of Joonam University Hospital. • Lee Yeon-soo as Kim Eun-hye : Hye-na's mother. She used to be Joon-sang's girlfriend, but they broke up after his mother's rejection. • Yoo Yeon as Laura Jung
Park family •
Kim Jung-nan as Lee Myung-joo : A former resident of SKY Castle, she committed suicide after her son ran away to his secret girlfriend. •
Song Geon-hee as Park Young-jae : Myung-joo's son. After his mother's death, he wants to reconcile with his father, Park Soo-chang. • Yu Seong-ju as Park Soo-chang : Myung-joo's husband and Young-jae's father, formerly working at Joonam University Hospital. At first, he is harsh and abusive towards Young-jae, but after his wife's death he wants to reconcile with his son. • Lee Yun-seol as Lee Ga-eul : Young-jae's girlfriend. She is 6 years older than him and was hired as a housekeeper by Myung-joo, coming from a low-class family. Myung-joo disapproved of her status and her relationship with Young-jae. •
Kwon Hwa-woon as Lee Choong-sun •
Shin Cheol-jin an elder at Soo-chang's cabin. : Like Soo-chang, he also has a son that committed suicide after being admitted to SNU with the aid of Kim Joo-young.
Special appearance •
Im Chan-mi as herself (ep. 3–4) ==Episodes== ==Production==