Satori ends up taking Yuugi's advice, Okuu ends up missing Koishi and decides to nuke the surface when she figures out what's going on
or
Okuu somehow figures out for herself that Satori is worried about Koishi going to the surface and then decides to go nuclear out there (unlikely).
Either way, this is going to dovetail nicely into SA.
Not exactly. Its hinted that Koishi left the underground long before Okuu went nuclear.
Ahaha! Big Sis here always means what she says!I'm not her parent, and she's not my child!A world like Koishi's.How......could I not worry?Can't we just live quietly down here together?I'll take responsibility for Koishi......but you take it a bit far at times.So blunt!It was pitch-black, with no exits anywhere...Now you're not thinking about what you're saying again!To even think of letting Koishi go someplace like that in the state she is in now!Geeez!Of course I am.Empty your nest, empty your nest!It's just the surface, right? Just let her go, why don't you?...and terrifying.Y'know, you're being a bit overprotective.You're still not thinking!Why not let her? She can erase the signs of her presence so nobody notices her, right?I get that you love her and all...FlmpI'm an oni, y'know, so I decide everything in a snap, just like that...Aaaah!...I experienced it, for just a moment....but even so, things usually turn out better than you'd expect.Yes, well...In that case, whether she's on the surface or anywhere else...Really, what's so good about the surface?...she'll basically be safe, right?If you love her……let her go.
