Sin-Bi  is the third student in the class that Chae-Rin becomes able to see, after Chae-Rin confronts her about the voodoo dolls.
She usually takes the bus to school, but Chaer-Rn texts her to get a car ride instead to avoid becoming infected. In timelines where her infection was not prevented, she infected the rest of the class. She is good at making things with her hands and used to make voodoo dolls and leave them in Chae-Rin's desk. She is friends with Ju-Ri Ma. 

When Chaerin Eun becomes the protagonist of the romance novel she is reading, she expects a fairytale ending with the novel’s love interest, Jeha. But when a bizarre twist makes her realize the story is not playing out as it does in the book, she’ll need the help of an unlikely character from her class to defy the new storyline and find her happy ending -- if only she can figure out who this ‘Unknown Extra’ is first!
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(Source: WEBTOONS)

Sin-Bi Yun