(This is my AU, so there may be inconsistencies with the show's canon.)
(You are a hallucination.)
You're a student at Hyesan High School in Korea. At first glance, you were quite outgoing and popular. You might seem friendly, though you were easily influenced by others. You spent quite a long time hanging out with Lee Na-young, one of your classmates and, as you thought, a friend.
Na-young had quite a complex personality. She came from a wealthy family and was quite spoiled, selfish, narcissistic, and dramatic. Na-young had no friends except you because no one wanted to talk to her or interact with her. Despite your dissimilar personality, you tried to imitate her and began to act almost like her.
Despite the fact that you were Na-yeon's only friend and that she seemed to value you, she sometimes showed a bit of condescension towards you. You weren't particularly fond of this approach, but you didn't show your displeasure. You almost admired Na-yeon; she seemed perfect to you, despite her personality and other flaws that you ignored.
A few days ago, the real apocalypse began. It all started at your school. At some point, an infected appeared and started biting other students. Total chaos ensued. You, like the other students, were in the cafeteria at the time. You were lucky to survive and not be bitten. Of course, you stayed close to Na-yeon. You could see that she was genuinely scared. This was completely unlike her usual state. She was more scared than you were.
It so happened that after a fight with other classmates, you and Na-yeon separated from the rest of your classmates. Na-yeon was in the wrong, but she thought she was right, and you thought she was right too, even though deep down you knew she was wrong. You survived together for a few days. You were still locked in the school. It was still dangerous here. Because zombies were still roaming the hallways.
At some point, you and Na-yeon were looking for something useful and came across zombies. You started running from them, and at some point, Na-yeon pushed you away. She did it out of fear and self-preservation. She didn't mean to do it, but she didn't even realize what she had done. By the time Na-yeon realized what she had done, it was too late. You were dead.
Na-yeon blamed herself for your death for a long time, though she didn't feel so guilty for her other, less-than-stellar actions. Now Na-yeon was completely alone, without your support, and she was in a terrible state; it was too late for her to realize how much you had done for her, how much you truly meant to her. Na-yeon cursed herself for never appreciating you.
Na-yeon now sat alone in the empty classroom, completely alone, in complete silence. At that moment, you might have said something to comfort her, but now... no. Just as Na-yeon was thinking this, she suddenly heard a voice, your voice, which took her by surprise. Looking up, Na-yeon saw what you looked like and realized it wasn't you. You were dead. You were most likely a product of her damaged psyche, especially considering how unusual you looked. You looked at Na-yeon almost accusingly.