Jayhun is a young man in his mid 20s. He dresses professionally. He has light brown hair kept in a spiky pattern and needs glasses.
Jayhun is a devout Christian, which other apartment residents find odd. However, Jayhun is very tolerant of other beliefs. Through Jayhun's faith, he is able to work up the courage to do things that would otherwise terrify him. He is also very kindhearted towards others, even towards Hyun Cha who is infected. He was implied to be attracted to Jisu, as she was the last person Jayhun thought of immediately before death.
Jayhun was a resident of the Green Roof Apartments when the monster apocalypse broke out. Thankfully, he was trained in kendo and had a keepsake katana sword to defend himself. He warned Jisu Yoon of the Blind Monster's attack and managed to slice part of its head off. Since then, the two have been bunking out in room 1107 for safety and supplies. They then went to fight the Steroid Monster to save Hyun Cha. Shortly after, the trio venture out again to save some children. Jayhun tricks the Steroid Monster into charging out of the apartment. He meets up with the rest of the group at Dusik Hahn's apartment. He spends a few nights there while Dusik outfits him with a shield that has a built in strobe to blind monsters. Eventually, he and Jisu decide that the building is not safe and they need more supplies. They, along with Hyun, plan to leave the building to find another safe zone. However, they are attacked by the Tentacle Monster, who nearly overpowers the trio. A rescue party from the first floor arrives to save them.
After being saved, Jayhun participates in the vote to accept Hyun. Later, Jisu confides with Jayhun her worries and the possibility of more infected among them. Jayhun is unsettled and realizes that they are not entirely safe in the lobby and plan to leave. Hyuk Lee, however, rather bluntly warns them the basement garage has been overran, trapping them inside for the time being.
After the Dusik gets rescued and power is restored to the elevator, Eun Lee notices that someone or something activated the elevator elsewhere. The culprit is Mr. Kim, the apartment's now-monsterfied security guard. Several survivors flee in terror while Jayhun stays back and fights. He attracts Mr. Kim's attention before it is able to kill Eun and manages to blind the monster. However, he tried tackling Mr. Kim but didn't even make him budge a little. Mr. Kim bats Jayhun to the ground. Jayhun realizes the end is near and notes the last person he is thinking of is Jisu. Mr. Kim then kills Jayhun by stomping on and crushing his skull.
His death, and the brutality of it, greatly traumatized Jisu and Hyun, and nearly caused Hyun to give in to monsterfication. Later, after his funeral, Wook Pyeon takes his sword.