In Parasyte: The Grey, Detective Kim Chul-min and Su-in share a close bond, which is why Chul-min’s death breaks Su-in apart.
The attack on Su-in at the start of the series is reported to Detective Kim Chul-min, who immediately visits Su-in.
Chul-min and Su-in know each other well. When Su-in was a kid, she reported her abusive father to the cops. It was Chul-min who came to her rescue.
To this day, Su-in regards Chul-min as the person who saved her life, but Chul-min continues to insist that Su-in saved herself by making the right decision to report her father.
While Su-in looks up to Chul-min, he cares about her as if she is still that little kid. Hence, when Namil Police Station is briefed about the parasites, Chul-min grows concerned about Su-in, who was recently involved in a strange accident.
Clues leading to Su-in continue to appear. Eventually, Chul-min starts hearing Su-in and Kang-woo’s story.
When Su-in is captured by Jun-kyung, Chul-min teams up with Kang-woo to rescue her, but the mission doesn’t end right for Chul-min.
The betrayal and death
Kang-woo informs Chul-min of a traitor in the Namil Police, who is none other than Detective Kang Won-seok.
Chul-min is with Won-seok during the mission to rescue Su-in. Won-seok drives Chul-min elsewhere and attempts to bring him on his and the parasites’ side after realizing that Chul-min is onto him.
Chul-min is reluctant. The leader of the parasites shows up. Despite Chul-min shooting at the host of this parasite, the parasite quickly leaves its host, cuts Chul-min’s head off, and takes over Chul-min’s body.
Su-in witnesses Chul-min’s death, which leaves her heartbroken. In Chul-min’s body, the leader of parasites spends a fair amount of time in Namil Police Station, on its way to its ultimate goal.
In the finale, it leaves Chul-min’s body in pursuit of the body of Namcheon’s mayor.