What's Wrong with Secretary Kim

Kim Mi-so's blind date (Ep. 3)

Kim Pil-nam

Young Madame Choi (Ep. 11–12)

Young Mi-so's mother (Ep. 10)

A childhood friend and former classmate of Lee Young-joon (Ep. 14)

Young Mi-so's father (Ep. 10)

Kim Ji-a

Lee Sung-yeon / Morpheus Bae Gang-yoo as 11-year-old Lee Sung-yeon

Sul Ma-eum

The woman at the reserved parking space (Ep. 12)

Oh Ji-ran, a famous model and onetime girlfriend of Young-joon (Ep. 1-3 & 16)

9-year-old Lee Sung-hyeon

Bong Se-ra

Park Yoo-sik

Bae Yun-sung, an intern staff.

Lee Young-joon / Lee Sung-hyeon Moon Woo-jin as 9-year-old Lee Sung-hyeon

Kim Mi-so Kim Ji-yoo as 5-year-old Kim Mi-so

Chairman Lee Go Se-won as young Chairman Lee (Ep. 11–12)

Young Chairman Lee (Ep. 11–12)

Madame Choi Lee Soo-kyung as young Madame Choi (Ep. 11–12)

And Lee Se-young as the staff who talked behind Kim Mi-so's back (Ep. 14)