mr. and mrs. yang (гет)

Arriving after being invited to a talk at Belobog's School of Social Sciences, Stelle checks in at the Goethe Hotel with Welt, only to realise that the form is asking her for a surname... which is something that she doesn't have.
Surely it's fine to just use Welt's surname as a placeholder, right? It didn't matter that Stelle has a raucous crush, and that they're alone, but what does matter is the insinuation taken by everyone around them.