Kim Seong-hee

Hospital department head. After the Yeongdeungpo explosion he departs by helicopter for the scene, orders Han Ju-won as chief to stay and guard the ER with Professors Min and An joining him, teases Han about kicking out interns again, and takes off.

Hospital doctor and colleague. Appears in the past intern conflict scene and later calls Woo Jin-han after the disaster broadcast, relaying that the chief wants him down immediately because the subway accident has thrown the hospital into chaos.

White-coat colleague with an ID badge. In the cafeteria he goads Woo Jin-han about Han Ju-won’s return and the old murder until Do-yeon stops him; in the flashback intern day he frets over ER assignment and dreads Han Ju-won.

Third-year emergency resident. Comforts crying interns, explains the chief’s perfectionism and strictness are teaching not hatred, offers to tip the chief if things are hard, and urges them to learn beside Han Ju-won.

Kim Seung-hwan’s guardian. Arrives with their daughter, tearfully asks about survival odds and her husband’s suffering, decides to stop resuscitation, then breaks down at the bedside.

Former ER chief now at the hospital. A flashback shows him nearly driving out all interns. After the Yeongdeungpo Line 1 train explosion news, he calls the situation the worst, is ordered to guard the ER while the department head leaves for the scene, activates Code Gray, and as disaster response commander briefs staff on mass casualties and crowd control.