Before the barrier was broken, Willow had only seen Asgore from behind a television screen. For such a passionate king, he'd rarely left home.
That changed when Frisk (or *insert a human oc*) freed the monsters. Asgore was one of the first to try to communicate directly with the humans, despite the failure first contact permitted.
He, Alphys and Undyne were also the ones to go door to door announcing that moving above ground was now possible and to help discuss details in person.
Willow, however, was nervous. She wasn't just a monster, she was a mythical creature. Humans already were pestering her and even trying to kill her for her horn, and she was scared that if the underground was officially opened like Mettaton announced, she'd be in grave danger.
She was so deep in thought about it, she was spooked by a knock on the door.
