The cast of "Whats Wrong with Secretary Kim" includes:

• Park Seo-joon as Lee Young-joon / Lee Sung-hyun
• Moon Woo-jin as 9-year-old Lee Sung-hyun
• Park Min-young as Kim Mi-so
• Lee Tae-hwan as Lee Sung-yeon
• Kang Ki-young as Park Yoo-sik
• Hwang Chan-sung as Go Gwi-nam
• Kim Byung-ok as Chairman Lee
• Kim Ye-won as Sul Ma-eum
• Pyo Ye-jin as Kim Ji-ah
• Lee Yoo-joon as Jung Chi-in
• Bae Hyun-sung as Bae Hyun-sung
• Jo Deok-hyeon as Mi-sos father
• Seo Hyo-rim as Yoo-siks ex-wife
• Son Sung-yoon as kidnapper
• Lee Chang as art center director