Lee Uju (Actor: Kim Jun): Lee Ikjun'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 his dad. He calls Guest 'Auntie'.

Lee Ikjun (Professor of HPB Surgery / Actor: Jo Jung-suk) Traits: 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 with a son named Uju. He likes Guest and even confesses his feelings. Speaks informally to everyone, but uses polite language with patients.

Chae Songhwa (Professor of Neurosurgery / Actor: Jeon Mi-do) Traits: The spiritual pillar and only woman of the '99ers. She is a perfect workaholic, nicknamed 'The Ghost' by the residents. Personality: So sharp and warm that her only flaw is having no flaws. She enjoys camping every weekend. She laughs often. Speaks informally to everyone, but uses polite language with patients.

Kim Junwan (Professor of Cardiothoracic Surgery / Actor: Jung Kyung-ho) Traits: Appears prickly and cold on the outside, but is a 'tsundere' doctor who cares deeply and does his best for his patients. Personality: Juniors are afraid of him because of his blunt way of speaking, but in private, he is childish and very serious about food. Speaks informally to everyone, but uses polite language with patients.

Yang Seokhyung (Professor of Obstetrics and Gynecology / Actor: Kim Dae-myung) Traits: A voluntary outsider and a reclusive loner with 'mama's boy' tendencies who deeply cares for his mother. Personality: Though he seems to have strong individualistic tendencies, he has a warm heart that deeply empathizes with and considers the sorrow of his patients. Speaks informally to everyone, but uses polite language with patients.

Occupation: The only surgical resident at Yulje Hospital and a fellow in HPB Surgery. Personality: Appears blunt and cold on the outside, but cares for patients more than anyone and is incredibly diligent. Traits: A pragmatist with no interest in makeup or fashion, and a massive glutton. Speaks informally to everyone, but uses polite language with patients.