Ms. Song was first shown when she was meeting Youngho outside of the 1-E classroom. She talked about Taehoon's attempted suicide, saying that she heard that he did it to grades, which she tells Youngho that there is more to life than grades. If Youngho tells her about Myung-gil's bullying, she will be shocked by Youngho's accusation and how Myung-gil was behaving well since the last time he was punished, which was a month long community service. She will tell Youngho that she needs to talk to him after school about his grades before she goes inside the classroom to start the day.
After Youngho enters the Coma, he will be chased the killer, who is dressed like Ms. Song with a knife or an ax. Once Youngho discovered the secret, it is shown that the killer was dressed like Ms. Song because the Shade used a mannequin and had it dressed in Ms. Song's clothes along with her pendant, so she could control the killer as in the image of Ms. Song.
This was further proved in one of the notes as it showed that when Ms. Song sleeps, she dreams of chasing giant cookies and pieces of cheese, which turned out to be Youngho and the others.
As to why the killer was dressed like her, The Noteman told Youngho that the girl in the pendant was a former Ghost Vigilante, who traveled into the Coma and encountered the Shade, which angered her greatly. So angry enough, that the Shade shaped the killer into their descendant's image to spite the former Ghost Vigilante.
During the second, Ms. Song tries to get inside the ambulance to go with Youngho to the hospital, but the paramedics told her that his parents were on his way and she needs to take care of her students. After Seho told her that Youngho will be fine, she said that she hopes so.
During the third ending, she holds the pendant in her hand and wonders why Youngho was holding it as she states that she needs to call her grandmother.
During the honest ending, she will congratulate Youngho on passing his test and classes as he turns in his exam.