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...
At first I was thinking that since she's a girl using "boku" she meant that Mahiro was the same but that seems a bit far-fetched. It'll be interesting to see where the artist takes it from here, hopefully it won't turn into some anti-climax next month.
It's kind of expected she'd be next-in-line. Notably though it's understood that there isn't an official successor so it can't be said for sure, as unlike when the previous Supreme Leader died when they knew it was coming and so had prepared the next-in-line.
Though of course we don't know the real situation on the matter, but something big will likely happen as they said satellite imagery has confirmed that his Train is currently at his Villa, and it's typically only used for rather major events (such as his visit to China), but was also used to transport the previous Supreme Leader's body when he died.