Main Cast
- Suzy as Go Hye Mi
- Kim Soo Hyun as Song Sam Dong
- Taecyeon as Jin Gook / Hyun Shi Hyuk
- Ham Eun Jung as Yoon Baek Hee
- Wooyoung as Jason
- IU as Kim Pil Sook
Extended cast
- Ahn Kil Kang as Ma Doo Shik
- Ahn Sun Young as Kang Oh Sun (Oh Hyuk's older sister)
- Ahn Seo Hyun as Go Hye Sung (Hye Mi's little sister)
- Park Eun Bin as 16 year old Hye Sung (ep 16)
- Lee Hye Sook as Song Nam Boon (Sam Dong's mother)
- Choi Il Hwa as Hyun Moo Jin (Jin Gook's father)
- Park Hyuk Kwon (박혁권) as Go Byung Jik (Hye Mi's father)
- Park Hwi Soon as Jin Gook's roommate (ep 1,2,5)
Cameos
- Jo Soo Mi (조수미) as herself (ep 1)
- Kim Hyun Joong as himself (ep 1)
- Nichkhun as Ri Ah's CF partner (ep 8)
- Hwang Chan Sung as Oh Sun's imaginary boy (ep 12)
- Lee Teuk as himself (ep 13)
- Eun Hyuk as himself (ep 13)
- Miss A as flash mob dancers (ep 16)
- 2AM as flash mob dancers (ep 16)
- Dal Shabet as Baek Hee's Kirin students (ep 16)
- Goo Joon Yup
Kirin's Teachers
- Uhm Ki Joon as Kang Oh Hyuk
- Lee Yoon Ji as Shi Kyung Jin
- Park Jin Young as Yang Jin Man (English teacher)
- Lee Byung Joon as Shi Bum Soo (Principal)
- Lee Yoon Mi as Maeng Seung Hee
- Baek Won Kil (백원길) as Gong Min Chul
- Bae Yong Joon as Jung Ha Myung (President) (ep 1-4)
- Joo Young Hoon (주영훈) as composing teacher (ep 11)
Kirin's Students
- Jun Ah Min as Jo In Sung (Jin Gook's friend)
- Jung Min Joo (정민주) as Jung Ah Jung
- Han Ji Hoo as Park Do Joon (sunbae)
- Yoon Young Ah as Lee Ri Ah
- Park Jin Sang (박진상) as Jun Tae San (sunbae)
- Kim Bo Reum as Ha So Hyun (sunbae)
- Bae Noo Ri (배누리)
