A reclusive high school student who lived in isolation after losing his family in a car accident. Having lost all hope in life, he moves into the Green Home apartments, only to get caught up in a sudden monster outbreak. Unlike others, he becomes a special entity who maintains human consciousness even as the monsterization process begins. Initially focused only on his own survival, he gradually grows into a person who sacrifices himself to protect the residents. His struggle to retain his humanity amidst fear and despair is the core of the story.

Though wary residents avoid him due to his intimidating appearance and blunt personality, he is actually a man with a strong sense of justice who seeks to protect the weak. With his massive physique and overwhelming combat prowess, he fights off monsters and serves as a reliable shield for the survivors. He carries a dark past and takes the lead in protecting people while battling his own guilt.

A drama-original character and a tough woman from a special forces background. Possessing excellent combat skills and cool-headed judgment, she acts more proactively than anyone for survival. She risks her life to find her missing fiancé and becomes connected to significant secrets as the seasons progress. She is a character who hides sadness and loneliness behind her strong exterior.

A former medical student who takes on the role of the de facto leader of the Green Home residents. He analyzes crisis situations with his brilliant brain and composure to establish survival strategies. Though sometimes misunderstood due to his cold, emotionless personality, he cares for the residents more than anyone. He is a character who constantly agonizes over making rational decisions in extreme situations.

A girl who dreamed of becoming a ballerina but lost her dream due to an injury. She uses rebellious and harsh language, but she actually has a warm heart. She grows close to Cha Hyun-su, and they become a source of strength for each other. While she pretends to be tough on the outside, she shows the side of an ordinary teenager who fears losing her family and friends.

A former rock band bassist with strong mental fortitude. She never loses hope even in difficult situations and encourages the residents. Despite carrying scars from her past, she refuses to succumb to despair and tries to move forward. She is a representative character who demonstrates whether humans can maintain their humanity until the very end.

A devout Christian and former Korean language teacher. Even in dangerous situations, he puts others first and demonstrates a spirit of sacrifice. Righteous and highly responsible, he earns the respect of the residents and never loses courage in moments of crisis.

A former nurse in charge of first aid and medical treatment. She has a calm and rational personality.