Mr. Kim was the security guard of the Green Roof Apartments complex. He eventually monsterfied and was killed.
Mr. Kim was working as the security guard at the Green Roof Apartment building. However, his work conditions were horrible. He was constantly berated by tenants at the slightest inconvenience and was also mistreated by the landlord. On one day, he was on the verge of turning into a monster, but a tenant unknowingly snapped him out of it. The tenant claimed she had a gift for him, which came from a surplus of food a relative sent. However, as Mr. Kim opened the cooler, it was revealed to be a cruel prank: The contents were actually rotten fish.
The human personality of Mr. Kim was not well known. He appeared to be saddled with most of the maintenance work of the building and is the recipient of cruel pranks, complaints, and emotional abuse by the apartment's tenants. He quietly complained about the tenants when alone.
As a monster, Mr. Kim's innermost desire appears to be vengeance. It remembers every bit of abuse and annoyance committed against it (some unintentional) and will shout out the related grievance before killing the victim. The monster often killed its victims in extremely gruesome ways, including completely crushing a skull. The known people the monster has a vendetta against are: Rotten Fish Lady (cruel prank), Jayhun (Mr. Kim doesn't like religion), and Jisu Yoon (Noise complaints).
Mr. Kim was the first person to greet Hyun as he moved in to the apartment and even gave him a bottle of milk. Hyun, however, treated him coldly. He bemoaned the distant nature and resumed work. As he did he showed signs of monsterification.
Some time prior to the full onset of the apocalypse, he locked down the entire building. However, he was so close to monsterification that when a tenant questioned him about it, he killed and absorbed him. He then went to the 15th floor to complete the lockdown, but remained there as the elevator broke from the weight of all the people he consumed.
When Dazik Han restored power to the elevator, Mr. Kim returned to the first floor. Initially, the survivors were happy to see him. However, Eun Lee realized Mr. Kim was actually a monster. Mr. Kim proceeded to kill the survivor who approached him and started attacking the other survivors. It was nearly able to kill Eun, but Jayhun managed to distract it. However, Jayhun was unable to do a thing to the monster and was brutally killed. Mr. Ahn arrived to protect Eun.
Mr. Ahn was eventually backed up by Hyun and Jisu. After a battle in which Hyun nearly turned into a monster himself, the Tongue Monster broke through part of the barricade. Hyun used this moment to attach its tongue to Mr. Kim, weakening Mr. Kim and letting the survivors incinerate the remains.
Mr. Kim was a human who gave in to monstrification. Unlike other monsters, Mr. Kim does not mutate heavily. He is able to take on the appearance of his normal self, albeit with an extremely deadpan facial expression. When enraged, his head twitches before his eyes and skin turn red.
Mr. Kim is the only monster seen using a tool or weapon, a weed whacker. The monster is able to turn its head 180 degrees. The monster absorbs its victims, which gives it an incredible weight, enough to make an elevator reach its weight capacity. The added weight also gives the monster extreme sturdiness. Hyun's electric spear, a tackle from Jayhun, and crossbow bolts from Mr. Ahn would not even make it flinch or move at all. It took the forced assistance from the Tongue Monster to finally weaken Mr. Kim enough to kill it. Upon death,its body loses the ability to remain merged with its absorbed victims and swells up as the bodies attempt to separate.
As with all monsters, Mr. Kim has an innate ability to sense monsterification in its targets. It lowered the weed whacker when it sensed Hyun was giving in to his monster, despite showing no visible sign of infection besides a blank face.