Main •
Bae Suzy as Seo Dal-mi •
Heo Jung-eun as young Seo Dal-mi : She dreams of becoming Korea's
Steve Jobs. She was born to a humble family and lives with her grandmother. However, she's also an adventurer who has a grand plan for herself. Her drive also came from being abandoned by her mother and sister at a young age. She also has experience in a wide range of
part-time jobs and is a person of unstable mentality. •
Nam Joo-hyuk as Nam Do-san •
Kim Kang-hoon as young Nam Do-san : Do-san is the founder of Samsan Tech. He was once the pride of his family as a math genius, but he is now shy and cannot even look people in the eye. After having had no business success in the past two years with his two programmer friends, he is almost ready to give up. Because of his name, Seo Dal-mi mistakenly believes him to be her 'cool first love' although they had never met. He decides to begin a startup in the hope of turning Seo Dal-mi's misunderstanding into a reality. •
Kim Seon-ho as
Han Ji-pyeong •
Nam Da-reum as young Han Ji-pyeong : A team leader at SH Venture Capital, his astonishing investment skills and sharp tongue earn him the nickname, "the
Gordon Ramsay of investments." Though he's prickly to most others, he's softer than anyone to one special person who granted him a great favor in the past. An orphan, at the age of 18 he was helped by Seo Dal-mi's grandmother and never forgot her kindness. He used the fake name of Nam Do-san to write letters to Seo Dal-mi at her grandmother's request. •
Kang Han-na as Won In-jae/Seo In-jae •
Lee Re as young Won In-jae : Won In-jae is Seo Dal-mi's older sister. She has everything society respects: a strong educational background, beautiful appearance, and money. She eventually realizes her background as a second-generation
chaebol is a weakness and does everything she can to create success on her own and be acknowledged for her skills.
Supporting Seo Dal-mi and Won In-jae's family •
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.
Samsan Tech •
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.
Nam Do-san's family •
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.
SH Venture Capital •
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.
Others •
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
Special appearances •
Kim Joo-hun as Seo Chung-myung (ep1) : Seo Dal-mi's and Won In-jae's father. Having had enough of being degraded at work, he decides to start his own business, which leads to his divorce and after a year, he manages to get a deal and it is implied that he inspired the name "Sandbox" during a chat with the owner who happens to be the current "Sandbox" CEO Yoon Seon-hak. On the very same day, while going home he dies in the bus due to brain injuries from an earlier car accident. •
Yang Dae-hyuk as Yoon Byung-soo (ep9-10) •
Yeo Jin-goo as Jang Young-shil (voice) / Hong Ji-seok (ep16) •
Lee Bo-young as woman at pub •
Moon Se-yoon as security guard •
Park Chan-ho as Nam Do-san's favorite baseball athlete ==Original soundtrack==