Main •
Park Seo-joon as Lee Young-joon / Lee Sung-hyeon •
Moon Woo-jin as 9-year-old Lee Sung-hyeon :Vice-chairman of Yumyung Group. He is handsome and capable, but his narcissism makes it difficult for Mi-so to work with him. He becomes aware of his feelings towards Mi-so after realizing that he might lose her. He can be jealous of Mi-so when she is with his brother or with someone other than him. At one point in his life, he suffered
PTSD. •
Park Min-young as Kim Mi-so • Kim Ji-yoo as 5-year-old Kim Mi-so :A highly skilled secretary who has worked with Young-joon for nine years. She soon reciprocates Young-joon's feelings after realizing that he was a part of her childhood.
Supporting ;Lee Young-joon's family •
Lee Tae-hwan as Lee Sung-yeon / Morpheus • Bae Gang-yoo as 11-year-old Lee Sung-yeon :Young-joon's older brother and a famous author. He had a crush on Mi-so. •
Kim Byeong-ok as Chairman Lee •
Go Se-won as young Chairman Lee (Ep. 11–12) :Young-joon and Sung-yeon's father. Chairman of Yumyung Group. •
Kim Hye-ok as Madame Choi :Mi-so's second oldest sister who is a urologist. • as Kim Young-man :Mi-so's father who is a rock musician. ;Yumyung Group •
Kang Ki-young as Park Yoo-sik :President of Yumyung Group, the second-in-command after the vice-chairman. Young-joon's best friend back from university. He is Young-joon's confidant and provides him with much advice drawn from personal experience. He is still in love with his ex-wife. •
Hwang Chan-sung as Go Gwi-nam :A section head in Chi-in's team, who initially coveted the open position of secretary to the vice-chairman, and initially had a crush on Gwi-nam. • as Jung Chi-in :A team leader in the vice-chairman's office, who frequently organizes after-work gatherings. • as Lee Young-ok • Choi Na-mu as woman at the Spanish Embassy party (Ep. 1) • Shin Joo-yeon as Kim Mi-so's friend (Ep. 1) •
Park Byung-eun as Park Byung-eun, Kim Mi-so's blind date (Ep. 3) • Lee Do-yeon as a reporter who interviewed Morpheus (Ep. 4) • Lee Chang as center director of the Art Center (Ep. 4) • as the host of the book concert (Ep. 8) •
Jung Soo-young as an agency staff (Ep. 9) •
Mihal Ashminov as a chef at a restaurant (Ep. 9) •
Seo Hyo-rim as Choi Seo-jin, Park Yoo-sik's ex-wife (Ep. 9, 11–12 & 16) •
Lee Min-ki as young Mi-so's father (Ep. 10) •
Jung So-min as young Mi-so's mother (Ep. 10) •
Kim Ga-yeon and
Lee Se-young as the staff who talked behind Kim Mi-so's back (Ep. 14) •
Lee Byung-joon as a wedding dress designer (Ep. 16) ==Production==