Name: Seol Kang-woo Date of Birth: April 12, 1988 Characteristics: Formerly a member of the Mangnani Gang but was betrayed. Ex-Marine. Has two prior convictions for violence. Has an older sister, Kyung-hee, and a younger sister, Jin-hee. He is currently being hunted by the Yeongdeungpo Gang after a failed attempt to take out one of their executives.

Name: Jeong Su-in Date of Birth: October 19, 1995 Characteristics: Works at Al-Mart after graduating high school. Estranged from her mother. Her father is currently serving a prison sentence. She trusts and follows Detective Kim Chul-min, who arrested her father, like a parent. When a customer attacked her, a parasite attempted to take over her body to survive; due to the severity of her injuries, the parasite could only successfully occupy the right side of her face.

Name: Heidi Characteristics: The parasite inhabiting the right side of Jeong Su-in's face. Seol Kang-woo gave her the name Heidi. Because the host integration is incomplete, she can only maintain her transformed state for a maximum of 15 minutes.

Name: Choi Jun-kyung Characteristics: Team Leader of 'The Grey' strike force under the Crisis Management Center of the National Police Agency. She refers to parasites as pests and viruses that must be eradicated. She lost her husband to a parasite.

Name: Kim Chul-min Characteristics: Team Leader of Violent Crimes Team 3 at Namil Police Station in South Chungcheong. A 'goose father' who sent his wife and son to the Philippines three years ago. Jeong Su-in is like a daughter to him.

Name: Kang Won-seok Characteristics: Detective (Sergeant) in Violent Crimes Team 3 at Namil Police Station. His true identity is a spy for the police, serving as a follower of Kwon Hyuk-ju's Saejin Church.

Name: Kwon Hyuk-ju Characteristics: The strongest among the parasites. He is the pastor and leader of the Saejin Church, a collective of parasites.