Main •
Yeon Woo-jin as Eun Hwan-ki, the cold and timid CEO of Brain public relations firm. He avoids people as much as possible by hiding out in his penthouse office or covering himself in a black hoodie. • as young Eun Hwan-ki •
Park Hye-su as Chae Ro-woon, an outgoing rookie employee who is surprisingly silent at home despite her extroverted nature and excellent communication skills She aims to take revenge on her boss for the death of her sister. •
Yoon Park as Kang Woo-il, the warm and confident, yet sensitive vice-chairman of Brain public relations firm. He is Hwan-ki's close friend and fiancé of Eun Yi-soo. • Lee Tae-woo as young Kang Woo-il •
Gong Seung-yeon as Eun Yi-soo, Hwan-ki's sister who serves as chaebol heiress and is introverted like him. She has mental illness that was initially instigated by her father's long verbal and emotional abuse of her older brother. • as young Eun Yi-soo
People in Silent Monster •
Ye Ji-won as Dang Yoo-hee, a working mother •
Jun Hyo-seong as Kim Gyo-ri, an employee who has crush on Se-jong •
Heo Jung-min as Eom Sun-bong • Han Jae-suk as Jang Se-jong, a new employee
People in Brain PR •
Stephanie Kim as Director Park • Jung Yi-yun as Assistant Jung • Woo-il's fan club and the elite of Brand Promotions AE • Hwang So-hee as Assistant Lee • Woo-il's fan club and the elite of Brand Promotions AE
Others •
Kim Eung-soo as Eun Bok-dong, Hwan-ki's father •
Kim Ye-ryeong as Park Ae-ran, Hwan-ki's mother •
Lee Han-wi as Chae Won-sang, Ro-woon's father •
Kim Mi-kyung as Ro-woon's mother •
Han Chae-ah as Chae Ji-hye, Ro-woon's older sister and Kang Woo-il's lover •
Lee Kyu-han as Woo Gi-ja, Ro-woon's neighborhood brother and a reporter •
Jang Hee-jin as Seo Yeon-jung, Hwan-ki's first love
Special appearances •
Kim Junsu as Top Star (Ep. 1) •
Choi Byung-mo as a presenter • Kim Ki-doo as a man in the restroom 1 (Ep. 2) /
Chinese restaurant food-delivery • Jo Hyun-shik as a man in the restroom 2 • Choi Joon-ho as a man in the restroom 3 • Song Yung-jae as Won-sang's friend •
Woo Hyun as Won-sang's friend • Seo Byung-sook as White Cloud Nursery School Director • Yoo Gun-woo as Secretary Kwak, Won-sang's secretary • Choi Eun-ho as Il-ho • Seo In-sung as Lee-ho • Lee Han-na as Si-yun • Geum Kwang-san as a people in sauna • Ji Dae-han as an advertiser for the Opera •
Kim Hye-eun as Hwan-ki's psychiatrist •
Kang Nam-gil as Brain security •
Kim Byung-man as Scrubber in Korean Spa (Ep. 2), Choir conductor (Ep. 4) and Orphanage Event MC (Ep. 6) •
Park Yeong-gyu as Actor Hwang Young-kyu •
Heo Young-ji as Young-kyu's daughter (Ep. 3) • Kim Dae-hui as Young-kyu's manager •
Park Sang-myun as Master Jin (Ep. 4) •
Choi Dae-chul as Restaurant Owner (Ep. 4) •
Lee Byung-joon as Rose Airlines CEO (Ep. 7) •
Kim Ji-seok as Yu-hui's husband • Han Suk-joon as Yeon-jung's first love. Also a Kookmin MC ==Production==