Seo Jun-young

Kang Tae-joon (32 years old) Choi Min-young as young Kang Tae-joon (12 years old)

..... Yoo Min-woo

Shin Woo-hyuk

Lee Ki-hyun / Cha Sung-tan Shin Ki-joon as young Cha Sung-tan

Ha Seung-ri/Minami Nanba, the leader of Dormitory 2 and track team captain with strong leadership skills who later falls in love with Seol Ha-na.

Prince Gwangpyeong

Lee Kang-so

Chun-woo

Kim Tae-yeol, Yoo-jin's ex-boyfriend

Jung Dong-min

Song Ji-min

Young Oh-soo

Kim Tae-hoon (Young-hoon's younger brother)

Park Jae-in (Tae-in's half-brother)