Futami Shouko exists outside the normal rules of space and time—she's essentially transcended her physical form entirely. Even before that happened, she could move freely through both time and space whenever she wanted. She's aware of a coming catastrophe called the "Upheaval" and knows the Kirihara brothers are going to be central to it. That's why she's been watching over them and subtly guiding their path. No matter when or where she appears, she's always wearing her school uniform.
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