This is a fanfic based on Nagisa's and Kayano's life together after the death of Koro-Sensei.
Yeah... he just HAPPENS to be available every time you want him to be, yeah... totally just a coincidence
Koro Sensei wasn't an alien he was a mutated scientific experiment
(dream demon response)
I'm assuming that what happened to them after isn't going to be the same here.