Main •
Lee Hyun-woo as Kang Han-gyul/K :A former bassist who quit the rock band Crude Play right before they debuted after losing his confidence to perform. He now works behind the scenes as its talented music composer and producer under the name "K". By chance, Han-gyul meets So-rim and begins to fall for her whilst hiding his true identity. •
Park Soo-young as Yoon So-rim :A nineteen-year-old high school student with a beautiful voice. She is scouted by a music CEO and forms the band
Mush & Co with her two childhood friends. So-rim's parents died when she was young, and she lives with her grandmother who runs a store called "Happy Fruits and Vegetables". While helping her grandmother by delivering orders to customers one day, So-rim accidentally meets and falls in love at first sight with Han-gyul. •
Lee Jung-jin as Choi Jin-hyuk, director of Sole Music who has been in love with Yoo-na from the beginning and has been waiting for her. •
Lee Seo-won as Seo Chan-young :A bass player who replaced Han-gyul in
Crude Play who feels that he has never been fully a part of the group. Chan-young is resentful of Han-gyul and later becomes the producer for So-rim's band and begins to fall for her. •
Hong Seo-young as Chae Yoo-na :Han-gyul's ex-girlfriend, a singer who is criticized by the media for using her body to gain fame. Even after they broke up, her feelings for Kang Han-gyul have never gone away.
Supporting Crude Play band members •
Kim Sung-joo as Yoo Si-hyun (Leader/Vocalist) • Shin Je-min as Lee Yoon (Guitar/Keyboard) •
Jang Ki-yong as Ji In-ho (Drummer/Rapper)
People around Han-gyul •
Choi Min-soo as Kang In-woo, a street singer who is Han-gyul's father.
People around So-rim •
Im Ye-jin as Kim Soon-hee, So-rim's grandmother, who owns a store called Happy Fruits and Vegetables. •
Song Kang as Baek Jin-woo, So-rim's childhood friend and the guitarist of the band Mush & Co. He has a long-standing crush on So-rim. • as Lee Kyu-seon, So-rim's childhood friend and the drummer of the band Mush & Co. •
Kim In-kwon as Bong Won-bin, So-rim's homeroom teacher who helped So-rim and her childhood friends become famous. • as Lee Se-jung, So-rim's classmate and rival who is a long-time fan of Chan-young.
People around Jin-hyuk •
Park Ji-young as Yoo Hyun-jung, CEO of Who Entertainment who has been in love with Han-gyul's father since she was young. • Lee Ha-eun as Yeon Soo-yeon, an employee of Sole Music who likes Si-hyun.
Others • Lee Yu-joon • Kim Min-seok as Ku Joon-seok •
Yang Kyung-won as Kim-eun • Park Seon-hee as Kyu-seon's mother • Kim Seo-young • In Sung-ho • Choi Nam-uk • Choi Na-moo • Jung Yu-an • Min Kyung-jin • Kang Woo-je • Yoon Eun-byul • Yu Ji-hyuk • Choi Ji-an • Yu Ji-yeon • Son Yoon-ju (Voice appearance) • Seo Dan-woo • Jo Young-seon • Lee Young-rae • Kyung Kyu-min • Lee Ho-seok as member of a band • Park Jae-seok as member of a band • Kim Soo-hyun as member of a band • Han Chan-young as member of a band
Special appearances • Baek Soo-ryun • Park Hye-na •
Park Hae-soo • Kim Young-pil •
Momoland as girl group from survival show "Produce 20" (Ep. 6) • INX, boy group from survival show "Produce 20" (Ep. 6) •
Kim Yong-gun as Chairman of Who Entertainment and Hyun-jung's father (Ep. 7) == Production ==