Chae Do-yeon

55-year-old man transferred after office chest pain, anterior MI on ECG, and no response to NTG at hospital A. He still has pain and dyspnea, asks staff not to tell his family because of his high-school daughter and tired wife, but Han refuses. He later goes into V-fib requiring CPR.

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.

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.

Elderly patient lying in a ward bed with monitor leads attached; Han Ju-won touching those leads triggers Woo Jin-han’s intrusion.

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.

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.