Main •
Bae Suzy as Go Hye-mi • Lee Joo-yeon as young Go Hye-mi (Ep. 2 & 5) :She originally wanted to become an opera singer but is forced to take up mainstream pop to pay off her father's debt to a gangster. •
Kim Soo-hyun as Song Sam-dong :A country bumpkin who is a music prodigy with a rare disease. He develops a one-sided crush on Hye-mi. •
Ok Taec-yeon as Jin-guk / Hyun Shi-hyuk •
Kang Yi-seok as young Jin-guk (Ep. 2, 5 & 10) :An aspiring dancer who plans on making his entertainment debut due to the contentious relationship with his father, the mayor, who hasn't acknowledged him as his son. Hye-mi's Love Interest. •
Hahm Eun-jung as Yoon Baek-hee :Formerly best friends with Hye-mi, the two become bitter enemies when Hye-mi betrays Baek-hee during an audition. •
Jang Wooyoung as Jason :An American-born dancer who plans on making his entertainment debut in Korea. •
Lee Ji-eun (IU) as Kim Pil-sook :She was discouraged from pursuing music because she was shy and overweight. She also has the gift of
perfect pitch.
Supporting •
Ahn Gil-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 younger 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-guk's father) •
Park Hyuk-kwon as Go Byung-jik (Hye-mi's father) • Jang Hee-soo as Kang Hee-seon (Baek-hee's mother) • Park Hwi-soon as Jin-gook's roommate (Ep. 1–2, 5)
Teachers in Kirin High School •
Um Ki-joon as Kang Oh-hyuk •
Lee Yoon-ji as Shi Kyung-jin •
Park Jin-young as Yang Jin-man •
Lee Byung-joon as Principal Shi Bum-soo •
Lee Yoon-mi as Maeng Seung-hee • Baek Won-kil as Gong Min-chul •
Bae Yong-joon as President Jung Ha-myung (Ep. 1–4) • Joo Young-hoon as the composing teacher (Ep. 11)
Students in Kirin High School • Jeon Ah-min as Jo In-sung (Jin-gook's friend) •
Joo as Jung Ah-jung • Han Ji-hoo as Park Do-joon •
Yoon Young-ah as Lee Ri-ah • Park Jin-sang as Jun Tae-san •
Han Bo-reum as Ha So-hyun •
Bae Noo-ri as Han So-ri (Ep. 6, 9, 12–13)
Special appearances •
Sumi Jo as Sumi Jo (Ep. 1) •
Kim Hyun-joong as Kim Hyun-joong (Ep. 1) •
Song Hae as variety show host (Ep. 2, 3) •
Jay B as Jason's back up dancer (Ep. 3) •
Young K as Jason's back up dancer (Ep. 3) •
Shownu as Jason's back up dancer (Ep. 3) •
Nichkhun as Lee Ri-ah's CF partner (Ep. 8) •
Koo Jun-yup as Koo Jun-yup (Ep. 9, 10) •
Hwang Chan-sung as Oh-sun's imaginary boyfriend (Ep. 12) •
Leeteuk as Leeteuk (Ep. 13) •
Eunhyuk as Eunhyuk (Ep. 13) •
miss A as flash mob dancers (Ep. 16) •
2AM as flash mob dancers (Ep. 16) •
Dal Shabet as Baek-hee's students (Ep. 16) ==Production==