I’ll spoiler this just in case
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From my understanding, the sister asked him to read a story her and her best friend made. When there was an accident where the ceiling caved in, it pulled him, both girls, and possibly his sunbae (who would have been there with him during the accident) into the world. His sister and her friend likely have their memories, while he’s slowly losing his. They’re freaking out because her brother paired himself with a ruthless, heartless gangster from their pov who he shouldn’t have interacted with until much later.

Lowkey confused about this whole story now