When Shi-mok pulls down Yoon's shirt, the complete tattoo can finally be seen. It shows the letters UDT, which refers to the Underwater Demolition Team of the South Korean Navy Special Forces.
The hero shows typical symptoms of acute stress disorder (psychological shock): narrowing the field of consciousness (when he can't drive), nightmares and flashbacks with the shocking event, avoidance of shocking memory (when he closes the picture of the shocking event), angry outbursts and search for a guilty person, physical symptoms such as difficult breathing or tears, slatternly appearance.