Either Mihari or one of her university classmates.
(Though, partly because "Nayuta"/Nayuta uses ぼく as her first person pronoun, and from name I'm inclined to think classmate of Mihari's.)
Or maybe the younger sibling of a former classmate of Mahiro's, but that explanation seems a bit... involved.
Or maybe she's a character very much prone to snooping/the offspring of someone in charge at the school...
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