Supporting
SangsangESang Publishing Company
Kim Joo-hun as Lee Sang-in
The CEO of SangsangESang Publishing Company, which publishes Moon-young's children's books.
Park Jin-joo as Yoo Seung-jae
The art director at SangsangESang Publishing Company who assists Sang-in.
OK Psychiatric Hospital Officials
Kim Chang-wan as Oh Ji-wang
The director of OK Psychiatric Hospital. Despite his use of unconventional methods, he genuinely cares for his patients and often succeeds at helping them get better.
Park Gyu-young as Nam Ju-ri
Park Seo-kyung as young Nam Ju-ri
A nurse and Gang-tae's co-worker at OK Psychiatric Hospital. She has an unrequited crush on Gang-tae, and is shown to be shy and easily jealous of others who are close with her romantic interests. She dislikes Moon-young, with whom she had a brief friendship in elementary school.
Kim Mi-kyung as Kang Soon-deok
A skilled chef at the hospital and Ju-ri's mother.
Jang Young-nam as Park Haeng-ja
The head nurse of the hospital.
Jang Gyu-ri as Sun Byul
A nurse with three years of experience who is Ju-ri's co-worker and friend.
Seo Joon as Kwon Min-seok
A psychiatrist at the hospital.
Choi Woo-sung as Oh Cha-yong
A young and careless caregiver who often sleeps at work. He is the son of hospital director Oh Ji-wang.
OK Psychiatric Hospital Patients
Lee Eol as Ko Dae-hwan
Moon-young's father, who is a patient at the hospital. He suffers from dementia and is bedbound. He was an architect who designed the Ko's family house, which is nicknamed the "Cursed Castle" after his wife Do Hui-jae's disappearance.
Kim Ki-cheon as Kan Pil-ong
A kind-hearted yet troubled Vietnam War veteran who suffers from post-traumatic stress disorder.
Jung Jae-kwang as Joo Jeong-tae
A patient content with life who was originally admitted to the hospital for alcoholism. He is Lee Ah-reum's boyfriend.
Ji Hye-won as Lee Ah-reum
A shy patient who was admitted for anxiety after escaping from her abusive ex-husband. She is Joo Jeong-tae's girlfriend.
Kang Ji-eun as Park Ok-ran
A mysterious fan of Hui-jae who torments Dae-hwan.
Joo In-young as Yoo Sun-hae
Ko Do-yeon as young Sun-hae (ep. 13)
A patient with dissociative identity disorder that she developed as a coping mechanism because of the troubled relationship with her father.
Others
Kang Ki-doong as Jo Jae-soo
Moon Gang-tae's best friend, who follows him and Sang-tae whenever they move.
Choi Hee-jin as Moon Sang-tae and Moon Gang-tae's mother
A single parent murdered over a decade ago under mysterious circumstances.
Woo Jung-won / Jang Young-nam as Do Hui-jae
A best-selling novelist and Moon-young's mother. She was emotionally abusive to Moon-young and is a big part of the reason why she is the way she is. She disappeared under mysterious circumstances over a decade before Moon-young meets Gang-tae again.
