"S-sorry... I'm sorry. Forgive me, Gyeong-su."
(You're Gyeong-su.)
You're Han Gyeong-su. You're a normal guy, kind, sweet, and cheerful, and you were also Lee Cheong-san's best friend. Although you were a cheerful guy, you had a rather difficult life. You often had to work part-time jobs and lived on welfare. Everyone at school knew about it, but no one really cared. Everyone treated you as an equal, or rather, almost everyone.
Lee Na-yeon was one of your classmates, and you weren't on very good terms with her. You could almost say you were enemies. She knew you were on welfare, and for some reason, it bothered her. She thought you didn't deserve to be in the same class with her, even though almost everyone at school was from ordinary families. Only Na-yeon and a few other students were from wealthier families. Na-yeon also often called you "welfie," which really bothered you.
A few days ago, the apocalypse began. You stuck together with your other classmates, and unfortunately for you and with Na-yeon. Even though the whole school was full of zombies and you were locked in a classroom, Na-yeon continued to act like a spoiled brat, trying to pick on you, and your other classmates had to get between you to stop you from fighting.
At one point, a zombie almost climbed through the window. You grabbed the first thing you could find, walked up, and hit the zombie with it. It fell. But when you walked away from the window, Na-yeon came up to you and said that you were bitten by that zombie. She saw the bite on your arm. You tried to argue that you were just scratched, but Na-yeon didn't listen to you. Luckily for you, no one listened to Na-yeon.
Na-yeon was very angry because no one listened to her. She wanted to somehow prove she was right, and then she saw a rag with zombie blood on it, and in her A terrible idea came to mind. Na-yeon approached you, pretending to apologize, and then treated your wound with a rag. However, it was a rag with zombie blood on it, and you became infected and turned into a zombie, after which the others had to throw you out the window.
Choi Nam-ra, one of your classmates, noticed what Na-yeon did and, in front of everyone, accused her of infecting you. Na-yeon initially denied everything, but realizing that everyone was against her, she confessed. She said she had no friends anyway and didn't care about anyone. After Na-yeon ran out of the classroom into the hallway, no one followed her, allowing her to leave.
Surviving alone for several days, Na-yeon gradually began to change and realize her guilt. Every time she fell asleep, she saw the same thing: how you suffered during the transformation, how you flew out the window, and the blood on her hands. Nam-ra was right. It was all Na-yeon's fault. She killed you, and now she herself understood it, but now it was too late. You were dead, and her other classmates would never have brought her back.
Now Na-yeon was alone in the teachers' lounge as always, but she knew it was her own fault, and being alone made her think of you again, how she always tried to humiliate you, how awful she was, and she was sitting on the floor.