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Lee Soo-hyuk as Han Ji-wook – A free-spirited and handsome detective assigned to a bizarre death case tied to the “S Line”. After wearing special glasses, he’s shocked to see dozens of red lines crisscrossing over his head and resolves to uncover the mystery. •
Arin as Shin Hyun-heup – The only person naturally born with the ability to see the “S Lines”. •
Lee Da-hee as Lee Gyu-jin – A mysterious high school teacher whose calm demeanor hides eerie smiles and secrets. • Jang Sun as Hyun-heup’s mother – Learns of her husband's affair with her sister after seeing her daughter's S-Line drawings. •
Kim Dong-young as Oh Dong-sik – Ji-wook’s junior and assistant detective. •
Lee Eun-saem as Kang Seon-ah – Ji-wook’s niece and Hyun-heup’s classmate. • Lee Kwang-hee as Joon-seon – Hyun-heup’s classmate and class president who secretly likes her. • Oh Woo-ri as Lee Kyung-jin – A fellow student at Hyun-heup’s school. • Lee Han-joo as Yoon Ji-na – A K-pop trainee attending the same high school. •
Nam Kyu-hee as Kim Hye-young – A classmate who bullies Seon-ah. • Kim So-young as Choi I-seo – Another bully. • Park Sung-il as Bang Seong-jin – Korean teacher who has an affair with student Hye-young. •
Woo Ji-hyun as Jung-woo – High school teacher who discovers his fiancée is cheating with a friend. • Lee Ka-seop as Oh Jung-min – A serial killer with a history of relationship violence, formerly a math teacher. • Park Ye-ni as Hee-won – Jung-min’s ex and Hyun-heup’s new neighbor. • Jung Jae-kwang as Bartender – Questioned by Ji-wook while investigating Jung-min. == Episodes ==