Supporting
Park Hoon as Sergeant Choi Woo-geun: A bomb disposal specialist and sniper. He is the only married member of Alpha Team and is nicknamed Snoopy.
Choi Woong as Staff Sergeant Gong Cheol-ho: The youngest member of Alpha Team, nicknamed Harry Potter.
Ahn Bo-hyun as Sergeant Lim Gwang-nam: An Alpha Team sniper, nicknamed Piccolo.
Kang Shin-il as Lieutenant General Yoon Gil-joon: Myung-joo's father. He is the main barrier in his daughter's relationship with Dae-young.
Kim Byung-chul as Lieutenant Colonel Park Byung-soo: The commander of the Taebaek Army in Uruk. An impulsive leader, he often clashes with Shi-jin over the latter's unpredictable behavior, which frequently causes Park to face difficulties with his own superiors.
Kim Min-seok as Private First Class Kim Ki-bum: A former pickpocket, he enlists in the army after being subdued by Shi-jin and Dae-young and is trained personally by the latter. His storyline illustrates an example of the military's power to reform and discipline individuals.
Onew as Lee Chi-hoon: A first-year thoracic surgery resident initially portrayed as naive and sheltered. Chi-hoon's storyline centers on his development as a professional; the Uruk earthquake is a turning point compelling him to confront his inadequacies and professional guilt. He is engaged to fellow resident Jang Hee-eun.
Lee Seung-joon as Song Sang-hyun: A general surgeon and Mo-yeon's senior colleague. Despite his disorganization and sense of humor, he is a skilled physician who plays a key role in the team's medical operations. He has unrequitedly pursued Ha Ja-ae for many years.
Seo Jeong-yeon as Ha Ja-ae: The head nurse of Haesung's ER, who is characterized as pragmatic and stoic. Alongside Song, she leads rescue efforts in Uruk. She has a hot-and-cold relationship with Song.
Park Hwan-hee as Choi Min-ji: The youngest nurse in the medical team. Known for her bright and "vitamin-like" personality, she provides comic relief and vitality to the team, even when innocently worrying about potential medical errors during high-pressure missions.
Hyun Jyu-ni as Pyo Ji-soo: A pathologist and Mo-yeon's confidante. She is always seen using a wheelchair, though the cause is not addressed.
Tae In-ho as Han Seok-won: The hospital chairman. Prioritizing ambition and power over medical ethics, he retaliates against Mo-yeon after she rejects his unwanted sexual advances by assigning her to the medical team in Uruk.
Park Ah-in as Kim Eun-ji: A professor appointed through nepotism who is characterized by her insolence and tendency to taunt or badmouth her colleagues.
Jo Woo-ri as Jang Hee-eun: An anesthesiology resident and Lee Chi-hoon's pregnant fiancée, who fears for his safety during the Uruk disaster.
David Lee McInnis as David Agus: An antagonist, a former Delta Force member and Shi-jin's comrade. After leaving the military in pursuit of wealth, he becomes a black market gang leader in Uruk. He represents an internal conflict for Shi-jin, who struggles with the necessity of killing a former ally who had turned into a villain.
Jo Jae-yoon as Jin Young-soo: The executive manager of the Uruk Power Plant. He is portrayed as unscrupulous and subservient to Agus, causing complications for the protagonists and inadvertently causing the M3 virus outbreak.
Ji Seung-hyun as Senior Lieutenant Ahn Jung-joon of the Korean People's Army: A North Korean elite officer. Initially a rival to Shi-jin, he becomes an ally after Shi-jin helps him expose a conspiracy involving a high-ranking superior, clearing Ahn of false charges and saving his life through a joint operation. Ahn later repays the debt by rescuing Shi-jin and Dae-young.
Jasper Cho as Daniel Spencer: An aid doctor with the Peacemaker and a skilled handyman who runs a hardware store.
Jeon Soo-jin as Ri Ye-hwa: A Koryo-saram nurse with the Peacemaker and Daniel's wife.
Lee Yi-kyung as Kang Min-jae: A worker at the Uruk Power Plant and a survivor of the earthquake. His resentment toward Lee Chi-hoon, following a failed rescue attempt, is central to the latter's psychological development.
Zyon Barreto as Fatima: A girl nicknamed "Red Rose". Saved by Mo-yeon and Shi-jin from a human trafficking ring, she serves as a catalyst for the former's ethical conflicts.
Elena Zhernovaia as Valentine: A waitress who assists the relief team.
Jun In-taek as Yoo Young-geun: Shi-jin's father and a retired master sergeant. His son's respect for him motivated Shi-jin to follow in his footsteps and commit to a military career.