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S Line kicks off with a wild and bold idea: Red threads appear above people's heads, connecting them to people s*xually.
This dark sci-fi K-drama revolves around suspense, mystery, and mostly social commentary with one important question: What happens when secrets are suddenly left out in the open?
But who are the actors that bring this crazy story to life? Let's break down the S Line cast and characters, and who plays whom in this bold, genre-bending thriller.
S Line cast and character guide
Lee Da Hee as Lee Gyu Jin
Lee Gyu Jin, played by Lee Da Hee, is introduced as a reserved high school teacher with no visible S Line over her head, setting her apart from everyone else. But there is more to the story.
She moves around the school with calm authority, but below that is something unsettling. When an incident takes place in the staff room, Gyu Jin walks right in with a strange calmness, helping sort out the issue at hand. Her facial expressions shift subtly throughout the series, giving her an unpredictable edge. Her mysterious energy lures the audience in, especially as she pretends to recognize people with secrets, adding another layer of suspense.
Lee Da Hee balances her dialogues with intensity, keeping the audience guessing about her actual motives. In just a few scenes, she turns her character into a major puzzle piece in S Line, adding to the eerie tone of the sci-fi drama.
Lee Da Hee is known for I Can Hear Your Voice and The Beauty Inside.
Lee Soo Hyuk as Han Ji Uk
Han Ji Uk is the detective at the center of S Line, and Lee Soo Hyuk plays him with a mix of charm and grit. His job is to investigate strange deaths, but things take a strange turn when the red lines start showing up.
Ji Uk isn't just solving murders, but he is chasing the mystery of the S Line itself. He crosses paths with Gyu Jin and other characters while trying to find the source of the glasses that let anyone see the lines. What sets him apart is how the character quietly reacts to everyone's exposed secrets while hiding his own. His role is heavy with tension, as even his niece gets pulled into the madness.
Lee Soo Hyuk is confused by the red line, but at the same time is obsessed with finding the truth behind it. He adds weight to every scene he is in, which only makes him out to be one of the show's most important characters.
Lee Soo Hyuk is known for Doom at Your Service and Pipeline.
A Rin as Sin Hyeon Hop
A Rin takes on the role of Sin Hyeon Hop in S Line, a teenage girl who has seen the red lines her entire life. For her, these lines aren't disturbing or shocking, but they are normal. However, this does not make them any less painful.
As a child, she found out about a family secret revealed through the lines, leaving her feeling alone. Her loneliness is sharp, and she hides this ability of hers from everyone around her. But when other people begin seeing the lines too, Hyeon Hop is forced to reveal the truth about the lines.
The moment glasses show up that let others see what she always has, she can no longer stay invisible. A Rin shows depth and control as a girl torn between staying hidden and doing what's right.
A Rin is known for The World of My 17 and Alchemy of Souls. She is also a member of the girl group, Oh My Girl.
Lee Eun Saem as KangSeon A
KangSeon A, played by Lee Eun Saem, is a high school student who has been pushed to the edge. As a victim of bullying, she has scars, and when the S Line glasses come to her, she sees an opportunity to pay back everyone who has hurt her.
What follows then is a change led strongly by rage, pain, and a desire to expose others the way she has been hurt. Her character is explosive, but also grounded with very real emotions. Lee Eun Saem delivers a performance that captures the intensity of a teen who has been pushed beyond her limits.
Lee Eun Saem is known for All of Us Are Dead and The Red Sleeve.
Jang Sun as Shin Hyeon Heup’s Mother
Jang Sun brings on devastation to the role of Hyeon Heup's mother. She sets the emotional tone for her daughter's journey. Though she doesn't dominate screen time, every scene she is in feels heavy. Her expressions carry guilt, loss, and something unspoken.
Jang Sun's film and theatre experience shows, offering a controlled performance that never overplays. Sun is known for Communication & Lies and A Killer Paradox.
Kim Dong Young as Detective Dong Sik
Kim Dong Young plays Detective O Dong Sik, the junior detective working alongside Han Ji Uk. He is sharp, loyal, and willing to step into uncomfortable situations to help his partner dig deeper into the case. While Ji Uk keeps things close to the chest, Dong Sik is more vocal, bringing energy and urgency to the investigation. His chemistry with Ji Uk adds dynamic contrast.
Dong Sik isn't just a supporting role, though. He is right there in the trenches as they chase leads, uncover secrets, and try to stay ahead of the growing danger. Kim Dòng Young leans into his character's commitment, turning a potentially side character into someone who matters.
Kim Dong Young is known for Room No.7 and My Strange Hero.
S Line is more than just a thriller, but it is a sharp look at secrecy, shame, and what happens when our private lives are no longer private. With a solid ensemble cast and bold characters, the series builds a world where nothing stays hidden for long.
The actors carry this bizarre premise with intensity, making S Line one of the most intriguing K dramas of the year.
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