Mouse

Jennifer Lee, Daniel Lee's sister and Han's former girlfriend

Jae-hoon Seo Woo-jin as younger Jae-hoon, a mysterious child born in 1995 whose family is murdered one day making him a key suspect in the case

Kim Joon-seong, Yo-han's classmate and friend

Oh Bong-yi, a confident and tough high school senior who lives with her grandmother and practices martial arts daily.

Ko Moo-chi, a detective who is willing to bend the rules to catch criminals. Song Min-jae as young Ko Moo-chi Suh Dong-hyun (ko) as teen Ko Moo-chi

Daniel Lee, a doctor of genetics and a criminologist

Kim Gat-nam, Bong-yi's grandmother

Ko Moo-won, Moo-chi's elder brother. He is a priest temporarily housed in the basement of a church. When he was young, he was saved from being killed by a serial killer, but has become disabled since. He is isolated from his younger brother Ko Moo-chi. Jo Yeon-ho (ko) as young Ko Moo-won

Song Soo-jung

Kang Deok-soo, a child sex offender

Shin Sang, a freshman detective working under detective Go Moo-chi

Sung Ji-eun, Dr. Han's wife

Jeong Ba-reum, a rookie police officer whose life changes after a confrontation with a psychopathic serial killer.

Choi Young-shin, a corrupt Secretary General in the Blue House who voted in favor of the Kill Psychopath Fetus Bill in 1995

Ba Reum's aunt

Dr. Han Seo-joon, a genius neurosurgeon doctor at Guryong Hospital

Choi Hong-ju, a talented production director of a reality TV program known as "Sherlock Hong-ju" at channel OBN.

Gu Dong-gu, a prison guard and Ba-reum and Chi Kook's friend. He was in the same class with Ba-reum and Chi-guk at a private academy.

Park Du-seok, head of the Military Manpower Administration Evidence Storage Team and former successful homicide detective

Bok Ho-nam, leader of the Mujin Bokbu Police Station Violent Team

Sung Yo-han, an emergency medical resident at Mujin Hospital. He is the youngest doctor in Korea to pass the state examination.