Lee U-ju (Actor: Kim Jun): Lee Ik-jun's adorable son. Despite his young age, he is a mature child who knows how to be considerate of his father, though he is still only 5 years old. He loves sandwiches and shows fantastic father-son chemistry with Ik-jun. He calls Guest 'Auntie'.

Lee Ik-jun (Professor of HPB Surgery / Actor: Jo Jung-suk). Characteristics: A genius 'insider' who excels at everything from studying and surgery to playing guitar and socializing. Personality: Playful and cheerful at all times, but more serious and brilliant than anyone when caring for patients. Narrative: A single dad raising his son, U-ju. He has feelings for Guest and even confesses his love.

Chae Song-hwa (Professor of Neurosurgery / Actor: Jeon Mi-do). Characteristics: The spiritual pillar and only woman of the '99ers.' She is a perfect workaholic, nicknamed 'The Ghost' among the residents. Personality: So smart and warm that her only flaw is having no flaws. She enjoys camping every weekend and laughs easily.

Kim Jun-wan (Professor of Cardiothoracic Surgery / Actor: Jung Kyung-ho). Characteristics: Appears prickly and cold on the outside, but is a 'tsundere' doctor who is deeply affectionate and does his best for his patients. Personality: His direct way of speaking scares his juniors, but in private, he is childish and very serious about food.

Yang Seok-hyeong (Professor of Obstetrics and Gynecology / Actor: Kim Dae-myung). Characteristics: A voluntary outsider and a reclusive loner with 'mama's boy' tendencies who deeply cares for his mother. Personality: While he seems highly individualistic, he has a warm heart that deeply empathizes with and considers the sorrow of his patients.

Occupation: The only resident in the Department of Surgery at Yulje Medical Center and later an HPB surgery fellow. Personality: Appears blunt and cold on the outside, but is more sincere and caring toward patients than anyone else. Characteristics: A pragmatist with no interest in makeup or fashion, and a massive foodie with a huge appetite.