Based on the novel with the same title published in 2011, the drama is about a group of passionate young people in their 20s who happen to live together. It drew parallels to the 2009 TV series Boys Over Flowers.
Eun Ha Won (Park So-dam) is a bright college student who dreams of becoming a teacher. Unfortunately, she loses her mother in a tragic accident, moves in with a cruel stepmother, and has no money for her education. One day, she helps an old man and as fate would have it, moves into a gorgeous mansion with three equally gorgeous men, who also happen to be billionaire cousins and heirs to the Kang family fortune. Between the rebel-minded loner Kang Ji Woon (Jung Il-woo), playboy money machine Kang Hyun Min (Ahn Jae-hyun) and the super-sweet singer Kang Seo Woo (Lee Jung-shin), Eun Ha Won finds herself in the middle of the hottest love quadrangle to ever befall a modern fairy tale princess. Rounding out the mansion's chaebol lifestyle are Lee Yoon Sung (Choi Min), who serves as the cousins’ handsome bodyguard, and Park Hye Ji (Son Na-eun), a gorgeous girl who has her own interests in the Kang family and is not above manipulating one cousin to get closer to another.
A smart, athletic high school student. Ever since her mother died, she lives a terrible home life because of the ill-treatment from her new stepmother and stepsister. In order to pay for her college tuition, she accepts an invitation to the Sky House and gets entangled with the Kang cousins. She met Kang Ji-woon at their mother's funerals, giving him her white ribbon hair pin and he gave her his white arm band. Their friendship was a brief one, so they couldn't recognize each other when reunited after 10 years.
The middle cousin, who went from an orphan living in poverty to a royalty overnight. Although he seems to be wild and rebellious on the outside, he is soft-hearted, extremely lonely and sensitive on the inside. At first, he tries to get closer to Ha-won in order to protect Hye-ji, who likes Hyun-min. However, as he gets to know Ha-won, he gradually falls in love with her. He originally met Ha-won at his mother's double funeral with Ha-won's mother, and gave words of comfort and his white arm band to her, while she gave him her white ribbon hair pin. It is suggested that their friendship was a brief one as they could not recognize each other when they were reunited after 10 years.
The eldest cousin, an arrogant playboy and smooth-talker who's used to getting whatever he wants in life. He firmly believes he can win any woman. At first, he appears to take an interest in Ha-won and pretend to be Ha-won's fiancé. However, he is still in love with Hye-ji, his childhood friend and his dead best friend's sister, but he hide the fact and chose to push her away due to a childhood trauma. As the time comes he realises his foolishness for pushing Hye-ji away and apologises for what he did and go back to his first true love. He has a troubled relationship with his mother due her constantly pushing him to be Haneul Group's successor, and for abandoning him when he was younger, only coming back to ask about his position.
The youngest cousin, a popular singer-songwriter and top star. He is a sweet, playful guy who is affectionate towards Ha-won whom he starts developing feelings for. He gives up his first love as he gets to know about Ha-won and Ji-woon's relationship. In the finale, it is hinted that he possibly ended up with Ja Young.
The secretary of Chairman Kang, and bodyguard to the three Kang cousins and Ha-won. He treats Ha-won really well and helps her settle into her new environment at Sky House. He protects Haneul Group by voting against the removal of chairman Kang, going against his mother's wishes. His mother is revealed to be Ji Hwa-ja, Chairman Kang's fifth wife.
A university student majoring in fashion design. She is a childhood friend of Hyun-min, and has liked him since they were kids. She tried using Ji-woon's crush on her to get closer to Hyun-min. She has a genuine and pure heart, but is constantly at battle with it because of Hyun-min constantly pushing her away. Especially after her brother's death.
Chairman of Kang Group and the Kang cousins' grandfather. He was reason behind Ji-woon not having his father present in his life, because he forcibly ended their relationship, and citing that people from different social classes should not be together. He almost does the same to Ji-woon and Ha-won, but relents and lets the couple be happy, not wanting history to repeat itself.
Chairman Kang's fifth wife, and the mother of Yoon-sung. She schemed to take over Haneul Group, by taking advantage of Chairman Kang being in coma, and put her son briefly as CEO of Haneul Group. Her efforts were foiled by Kang Hyun-min convincing 52% of the board against removing his grandfather, and also secretly by her own son. Chairman Kang divorces her, but relents and possibly remarries her again.
Ha-won's father. He works as dump-truck driver at construction site. He originally believed Eun Ha-won to not be his daughter, and thus hated not only her, but her deceased mother as well, thus kicking her out. His theory was put down by Kang Young-jin, and he tries to make amends with his daughter again. He also moves his second wife and step-daughter to the same small room that Eun Ha-won slept in at the beginning of the series, as punishment for their cruel treatment of his daughter
Choi Eun-kyung as Park Soo-kyung
Ha-won's stepmother. She works at a spa and dreams of getting her daughter rich and famous. She is eventually fired from her job for trying to offer her service as a spy to Madam Kang, in a vain and ill-thought out plan to have her daughter marry one of the Kang cousins and make her famous.
Ha-won's stepsister. She aims to be a top star one day and marry Kang Hyun-min. Her grades in school were poor, seeing as how she couldn't get into University, and is lazy and unemployed. She tries to switch her advances to Kang Seo-woo, but is rebuffed by him, by writing in her autograph book to "be nice to Eun Ha-won". She's childish and immature because her mother constantly spoils her and never made her lift a finger to work in her life, and thus having to rely on her mother for everything.
Ha-won's best friend who works part-time at the cafe. She met Eun Ha-won when she couldn't afford food, and Ha won gave her some of her almost expired food, they have been best friends since then She has a mother who works full-time, and a father who can't work yet due to recovering from cancer treatment. She is a fan of Kang Seo-woo, and possibly becomes his girlfriend in the series finale.
Shin Dong-mi as Han Jung-sun (Ha-won's mother) – She deceased before the main events of the show, Eun Ha Won's Father believed to be unfaithful to him, but was proven to be false. She died alongside Kang Ji-woon's mother trying to save her, and tried to give a keepsake ring to Kang Ji-woon, but ended up being mistakingly given to Eun Ha Won.
Jin Hye-won as Hye-ji's friend
Jung Young-joo as housemaid
Jo Kyung-hoon as Ji-woon's boss
Kim Kang-hyun as Seo-woo's manager
Lee Ah-hyun as Seo-woo's mother – She is very easy going, and somewhat childish, but still loves her son a great deal, and still very much in love with her deceased husband, Kang Seo-woo's father.
Park Kyu-jeom as Director Kim
Park Hyo-jun as customer in car center
Jo Eun-suk as Hyun-min's mother – cold and arrogant, she only cares about her son succeeding Haneul Group, constantly pressures him to stay as the main heir.
Gong Jung-hwan as Kang Young-jin – He was mistaken to be Eun Ha-Won's biological father, but turned out to be false. He tried using this false information to his advantage by getting money from Kang Ji-Woon to pay his debt, but was foiled by Eun Ha-won.
Na Jong-chan as Joon Soo (Hyun-min's friend) - Asks Hye-Ji on a date.
Kim Ji-sung as Ji-woon's mother – Deceased before the events of the show, she dies alongside Eun Ha-won's mother, and begged Ha-won's mother to give her ring to Kang Ji-woon. She didn't let Ji-woon's grandfather know him, for the fear that he would not be able to love someone without the worry of social status, and being ripped apart like her and Ji-woon's father were.
Kim Young-jae as Ji-woon's father – He was forcibly separated from Ji-woon's mother, by his own father, believing that people of different social classes should not be together. It is believed he died before he could ever find out about his son.