Supporting
Kim Hae-sook as Choi Won-deok
Seo Chung-myung's mother, and Seo Dal-mi and Won In-jae's grandmother.
Song Seon-mi as Cha Ah-hyun
Seo Dal-mi and Won In-jae's mother. Having divorced her husband out of impatience with his unstable employment, she ends up marrying the wealthy Won Doo-jung. She ultimately regrets leaving Dal-mi and In-jae's dad for Doo-Jung as he cheats on her but she finds it hard to give up the luxurious life which he affords her.
Um Hyo-sup as Won Doo-jung
Won In-jae's stepfather and the Chairman of The Morning group. He shows a ruthless drive to exert power over others in his drive for business dominance.
Moon Dong-hyeok as Won Sang-soo
Won Don-Jung's son, Won In-jae's stepbrother. In a power move, he becomes the Chief Executive Officer of the Morning group.
Yoo Su-bin as Lee Chul-san
A friend of Do-san since they were in university, he joined the company after resigning due to a ransomware that had taken control while he was in-charge and was told to pay 100 million won.
Kim Do-wan as Kim Yong-san
A friend of Do-san since they were in university, he had his own agenda for getting into Sandbox, relating to the death of his brother who was a CEO at Sandbox's 2nd cohort.
Stephanie Lee as Jeong Sa-ha
A former lawyer who quits her job to pursue something more adventurous. She speaks fluent English and intersperses English expressions even while speaking Korean with her teammates.
Kim Hee-jung as Park Geum-jung
Nam Do-san's mother.
Kim Won-hae as Nam Sung-hwan
Nam Do-san's father.
Jang Se-hyun as Nam Chun-ho
Nam Do-san's cousin.
Seo Yi-sook as Yoon Seon-hak
The CEO of Sandbox and SH Venture Capital.
Kim Min-seok as Park Dong-cheon
The assistant or secretary of Ji-pyeong and mentoring manager of Sandbox's 12th batch.
Jasper Cho as Alex Kwon
One of the mentors at Sandbox's 12th batch and the owner of 2STO, a Silicon Valley company.
Kang You-seok as Shin Hyeon
A computer programmer, Shin Jeong's twin brother.
Joo Bo-Young as Shin Jeong
A computer programmer, Shin Hyeon's twin sister.
Kim Ji-in as Seo-hyun
