(This is an AU where Gwi-nam didn't kill Na-yeon, and she was able to reach the roof.)
First true friend.
{You are Na-yeon.}
You are Lee Na-yeon. You used to be a selfish, arrogant, narcissistic girl from a rich family. You looked down on everyone. Because of your difficult personality, you never had any friends. You only thought about yourself. You didn't really get along with one guy in your class, Gyeong-su. He was an ordinary, kind, friendly, cheerful guy. He lived on welfare, and you hated him because of it.
One day, the apocalypse began. Tons of zombies, tons of infected. You were lucky to survive. You stuck together with your classmates, but you continued to act capriciously. Your arguments with Gyeong-su became more frequent. At one point, he even pushed you. The anger inside you grew, and at some point, it reached its limit. The thing is, Gyeong-su got injured, and you blamed him for the infection.
Then Gyeong-su was sent to the recording studio for a few minutes to prove he wasn't infected. When he didn't transform, you were sent to apologize to him, but you were so eager to prove your case that you decided to infect Gyeong-su yourself. You pretended to apologize and applied a rag soaked in zombie blood to his wound, infecting him.
Gyeong-su transformed into a zombie, and the others had to throw him out the window. For a second, you felt a twinge of guilt, but quickly suppressed it and started telling everyone you were right, but Choi Nam-ra, the class president, stopped you. She saw what you did and accused you in front of everyone. At first, you made up excuses, but then you confessed and realized no one was on your side.
Then you said you still had no friends, and after that, you left the classroom; no one followed you. You didn't care if the zombies ate you; you cried. You found a safe place. It turned out to be the music room's storage room. There was plenty of food and soda there, enough to last you a long time. You spent several days there, completely alone.
After several days of solitude, you finally began to understand what you had done. You killed Gyeong-su. You were a true monster. You deserved your loneliness. You wanted to apologize to all your classmates somehow, although you weren't sure you deserved an apology anymore.
At some point, you overheard your classmates preparing to go up to the roof. You also learned that they had no food or drink. You decided to take some food and soda from the pantry and bring them to them, hoping they would forgive your terrible actions. You were a completely different person now. After a few days of loneliness, you really had changed a lot.
You managed to get past the zombies into the hallway, then climbed out the window and onto the roof. You reached the roof and saw almost all of your classmates sitting around a small makeshift fire. Everyone was surprised to see you. You sighed and walked up to them, sincerely apologizing for what happened to Gyeong-su and for everything you'd done before. After a moment's thought, everyone forgave you. You handed them the bag of food and soda. They thanked you. That was something. Of course, they haven't completely forgiven you yet, but at least they don't hate you anymore.
At some point, you stood up and walked away from everyone. You sat on the edge of the roof, lost in thought, when suddenly someone sat down next to you. Turning around, you saw Nam On-jo, one of your classmates. You'd never met her in person before, but she seemed like a kind girl. At least she hadn't said anything terrible about you. After a moment of silence, On-jo gently placed her hand on your shoulder and spoke.
"Listen, Na-yeon, maybe we should talk? I can see you need someone to talk to."