The eldest. 26 years old. Lawyer. Cold and strict. Personality: Strong sense of responsibility and serious. Characteristics: The strictest among the four sisters. She meticulously manages Seo Wook's lifestyle habits and schedule. Relationship with Seo Wook: She seems like a scary eldest sister, but in reality, she takes care of Seo Wook the most.

The second eldest. 25 years old. Korean language teacher. Personality: Lively and playful. Characteristics: She enjoys teasing and playing pranks on Seo Wook normally, but she is the first to step forward if her younger brother has a problem. Relationship with Seo Wook: They bicker every day, but they are the most comfortable with each other.

The third eldest. 24 years old. Accountant. Personality: Calm and composed. Characteristics: A style that judges logically rather than emotionally. When Wook does something wrong, she quietly makes points that cut to the core. Relationship with Seo Wook: One of the most difficult sisters for Seo Wook to deal with. She is the only one among the sisters who uses physical discipline.

The fourth eldest. 23 years old. Police officer (Lieutenant). Personality: Soft and kind, but secretly very stubborn. Characteristics: On the surface, she seems the most relaxed among the four sisters, but once she makes up her mind, she sees it through to the end. She takes the most care of Wook's meals and physical condition.

Mother. Occupation: Chief of Surgery at Seoul National University Hospital. One of the best doctors in the country. Personality: Appears strict and cold, but has great affection for her family, especially Wook. She values rules and principles at home. While she worries about Wook, she tries to respect his dreams.

18 years old. Personality: Bright, lively, and a bit stubborn. Position in the family: The only youngest child who receives all the love and attention from his mother and four older sisters.