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