In the 2010 South Korean thriller I Saw the Devil, the narrative is built on a series of nested secrets, hidden identities, and clandestine motives. The characters often use secrecy as a weapon, leading to a catastrophic spiral of violence.
### **Kim Soo-hyun (The Protagonist)**
As an elite agent for the National Intelligence Service (NIS), Soo-hyun is a master of tradecraft, which he uses to keep his revenge plot entirely off the books.
* **The Tracking Device:** After capturing the serial killer Jang Kyung-chul for the first time, Soo-hyun does not arrest or kill him. Instead, he forces Kyung-chul to swallow a **GPS tracking capsule** containing a microphone. This is his most critical secret; it allows him to listen to Kyung-chul’s conversations and track his location in real-time, turning the killer into a pawn in a "catch-and-release" game.
* **Illegal Vigilantism:** Soo-hyun hides his activities from his own agency and the local police. He takes a "leave of absence" to hunt Kyung-chul, using his professional connections to secretly obtain files on suspects and medical supplies to treat his own wounds so he can stay in the field without detection.
* **The Final Trap:** In the climax, Soo-hyun sets a mechanical "guillotine" trap in Kyung-chul’s own basement. He keeps the nature of the trap secret from Kyung-chul until the very last second, when he reveals that he has invited Kyung-chul’s family to the house to unknowingly trigger the blade.
### **Jang Kyung-chul (The Antagonist)**
Kyung-chul’s entire existence is a secret kept from the public and the authorities.
* **The School Bus Driver Persona:** By day, Kyung-chul works as a seemingly mundane school bus driver. He uses this "respectable" job as a cover to scout victims and travel unnoticed through the city.
* **The Hidden Lair:** He maintains a secret, blood-stained workshop in a remote area where he dismembers his victims. This location remains a mystery to the police for years until Soo-hyun’s investigation unearths it.
* **Connection to Other Killers:** Kyung-chul hides his ties to a network of other sociopaths, including **Tae-joo**, a cannibalistic murderer. They share a "secret language" of violence and provide each other with safe houses.
### **Jang Joo-yun (The Victim)**
The murder of Soo-hyun’s fiancée, Joo-yun, is the catalyst for the film.
* **The Pregnancy:** Joo-yun was keeping the secret of her pregnancy from her father (a police chief) and had only recently shared it with Soo-hyun. In a devastating scene, she reveals this secret to Kyung-chul as a plea for mercy, hoping his humanity might spare her unborn child. He ignores it entirely.
### **Supporting Characters (Tae-joo and Se-jung)**
* **The Cannibalism:** Tae-joo and his girlfriend, Se-jung, live in a lavish home where they secretly practice cannibalism. To the outside world, they are a wealthy couple, but their basement holds the remains of their victims.
* **The Secret Affair:** There is a subtle, treacherous secret between the villains: it is revealed that **Se-jung has been having an affair with Kyung-chul** behind Tae-joo's back. This creates a tension of "dishonor among thieves" that Soo-hyun eventually exploits when he infiltrates their home.
### **The Police Department**
* **Squad Chief Jang (Joo-yun's Father):** While he is a lawman, he secretly provides Soo-hyun with the confidential list of suspects that allows the revenge mission to begin. He essentially bypasses the legal system to allow his son-in-law to act as an extra-judicial executioner, a secret that would ruin his career if discovered.