Lee Jong Suk as Park Hoon
– Goo Seung Hyun as Park Hoon (Child)
Jin Se Yun as Song Jae Hee / Han Seung Hee
– Seo Ji Hee as Song Jae Hee (Child)
Park Hae Jin as Han Jae Joon
– Kim Ji Hoon as Han Jae Joon (Child)
Kang So Ra as Oh Soo Hyun
– Shin Soo Yun as Oh Soo Hyun (Child)

Myung Woo University Hospital

Jun Gook Hwan as Oh Jong Yoo
Choi Jung Woo as Moon Hyung Wook
Nam Myung Ryul as Choi Byeong Cheol
Kim Sang Ho as Yang Jeong Han
Hwang Dong Joo as Geum Bong Hyun
Kang Tae Hwan as Oh Sang Jin
Lee Jae Won as Kim Chi Gyu
Uhm Soo Jung as Min Soo Ji
Han Eun Sun as Eun Min Se
Jung Hye In as Yeo Sun Hee

People outside the hospital

Chun Ho Jin as Jang Seok Joo
Jung In Ki as Kim Tae Seol
Park Hae Joon as Cha Jin Soo
Yoon Bo Ra as Lee Chung Yi
Kim Yong Gun as Hong Chan Sung
Hwang Bum Shik as Mr. Lim
Kim Ji Young as Jung Min
Sung Byung Sook as Lee Chang Yi’s mother
Won Jong Rye as Oh Sang Jin’s mother

Cameo

Kim Sang Joong as Park Chul
Lee Il Hwa as Park Hoon’s mother
Park Hee Bon as dr. Song Jae Hee
Gil Hae Yun as Kim Eun Hee (Oh Soo Hyun’s mother)
Lee Seung Hyung as presidential chief of staff
Zhang Liang as Sean Zhang (Jae Joon’s friend)
Kim Ji Eun as Oh Joon Gyu’s secretary
Kim Bo Mi as Kim Ah Young (Kim Chi Gyu’s younger sister)