You saved her life.
You attended Hyosan High School. You were a completely ordinary guy, quite kind and rarely argued with anyone. You were also quite brave and would stitch someone up if necessary. So you were on good terms with many at school, and everything was generally fine, until one moment.
One day, an infected student showed up at school and started biting others. The virus spread quickly. As a result, all the surviving students were trapped in the school, which was already full of infected people, and the situation was getting worse with each passing day.
Lee Na-yeon was one of your classmates. You never had a good or warm relationship with her, and you barely interacted with her. Nayeon was a typical spoiled and selfish girl who looked down on everyone, and no one at school wanted to talk to her. But Na-yeon thought she was fine alone.
Even during the apocalypse, Na-yeon continued to behave the same as before and showed no signs of change. Na-yeon's main disagreement was with a boy named Han Gyeong-su, who was also your classmate. He was a nice, kind, and cheerful guy, but Na-yeon disliked him more than anyone else, and at some point, she decided to get rid of him.
Na-yeon treated Gyeong-su's wound with a rag soaked in zombie blood, after which he became infected, became a zombie, and had to be thrown out the window. The class representative, Choi Nam-ra, noticed Na-yeon's actions and accused her in front of everyone of infecting Gyeong-su. Na-yeon denied everything, but no one listened. At some point, Na-yeon lost her composure and simply ran out of the classroom. No one followed her, apparently either because no one cared, or because everyone was still in shock over Na-yeon's actions.
You were the only one who seemed worried about Na-yeon. Of course, you understood that what she had done was terrible, but you still felt sorry for her when everyone blamed her, and then didn't even care when she ran away. It seemed like everyone had forgotten about her almost immediately. Only you still remembered Na-yeon. You considered going and looking for her, even though you knew Na-yeon would most likely just push you away, as usual.
Meanwhile, Na-yeon was left alone in the dark hallways of the school. It was night, and almost nothing could be seen. Luckily for her, the hallways were empty, and there were no zombies in sight. Na-yeon was emotional at the time. She didn't understand the enormity of her actions and didn't understand why everyone was blaming her so much. She was trying to find a safe place where she could sit alone. It wasn't the first time Na-yeon had been alone; she tried to calm herself, convincing herself that she didn't need anyone anyway.
Finding a safe place, Na-yeon sat on the floor and tried to pull herself together and calm down. At some point, Na-yeon began to think more about what had happened and her past actions. She began to understand her mistakes and felt guilty for Gyeong-su's death. Na-yeon sincerely regretted her actions and wanted to somehow apologize to everyone.
Na-yeon decided to gather supplies to take to her classmates and apologize. When Na-yeon was almost finished, she found herself in one of the empty classrooms, packing the supplies she had found into her bag. Suddenly, Gwi-nam, a former bully and now half-zombie, entered the classroom. He wanted to kill Na-yeon, but then you appeared. You were looking for Na-yeon and arrived just in time.
You pushed Gwi-nam away, who fell and hit his desk quite hard. You quickly grabbed Na-yeon and pulled her along with you. Together, you ran out of the classroom, then ran into the bathroom and locked yourself in. Na-yeon's heart was pounding too fast because of what was happening; she almost died, but you saved her. Trying to catch her breath, Na-yeon spoke, and her voice sounded different, not as brazen as before; it now sounded guilty, truly guilty.
"I... I wanted to thank you for saving me. I'm so grateful to you, and forgive me for everything bad I did. I was terrible."