Eom Jae-il is a 28-year-old male and a 1st-year OB/GYN resident at Jongno Yulje Hospital. Before becoming a doctor, he was a member of the idol group **'Hi-Boys' under the stage name 'Eom J'**. Despite his unique background as a former idol, he tries harder than anyone to adapt to hospital life. However, he struggled with academics and exams, having retaken the national licensing exam and ranked last during his internship. After being rejected by every other department, he finally landed in OB/GYN. He is the brightest, most cheerful, and affectionate among the three colleagues. He loves lightening the mood, befriends people easily, and empathizes well with others' emotions. He has a warm heart and cannot ignore those in difficult situations. However, his excessive passion sometimes burdens those around him or leads to unintended mistakes. Nevertheless, he is a character who recognizes his shortcomings and constantly strives to improve.

Kim Sabi is a 28-year-old female and a 1st-year OB/GYN resident alongside Eom Jae-il and Oh Yi-young. She is highly intelligent and academically brilliant. She deeply admires Professor Seo Jeong-min and was inspired to apply for OB/GYN after reading the professor's papers. Above all, she possesses incredible memorization skills and superior knowledge, making her overwhelmingly dominant in theoretical aspects among her peers. Conversely, her personality is very blunt, cold, and direct. Rather than sugarcoating words to suit others' feelings, she is the type to state the facts exactly as she knows them.

Oh Yi-young is a 28-year-old female and a 1st-year OB/GYN resident like Kim Sa-bi and Eom Jae-il. She is described as having an indifferent and blunt personality, rarely showing her emotions to others. She doesn't particularly like being at the hospital and maintains a fairly cynical attitude toward resident life. However, contrary to her cold and indifferent exterior, she is actually warm-hearted and very passionate about the people or work she cares for. Especially in situations involving patients, she doesn't avoid her role but ultimately steps up to take responsibility. Thus, her defining trait is being 'cold on the outside, warm on the inside.'

Pyo Nam-gyeong is a 28-year-old female and a 1st-year OB/GYN resident at Jongno Yulje Hospital. She is a peer of Oh Yi-young, Kim Sa-bi, and Eom Jae-il, starting her residency alongside them. On the surface, she appears sophisticated, confident, and values style. She has a great interest in clothes, makeup, and grooming, striving to maintain her style even in the midst of a busy hospital life. However, she isn't someone who only cares about appearances; she has a strong desire to be recognized for her skills as a doctor. She aims to be a physician who possesses both style and competence. Her personality is bright and social, and she mingles easily with friends. She has been close colleagues with Oh Yi-young since high school.

Gu Do-won is a 33-year-old male and the 4th-year Chief Resident of OB/GYN at Jongno Yulje Hospital. Having trained in OB/GYN for a long time, he is highly experienced and skilled, making him one of the first seniors the juniors seek out when facing difficulties. Consequently, he is nicknamed 'Class President Gu' by the professors and is regarded as a reliable senior by his juniors. His personality is fundamentally calm, kind, and composed. Rather than getting angry when a junior makes a mistake, he usually tries to understand the situation and help, stepping in personally to resolve problems when necessary.