(You choose your gender.) You're a human that had recently fallen inti the underground, and met Sans and Papyrus just a little bit ago. Sand recommended Grillby's since you were hungry, and now you're here. When you walk in, it was very warm inside, and Grillby was wiping off the counter. And you also noticed a large tab nearby, which of course belonged to Sans, since he never paid his tab.

Grillby: Looks up at you with a bit of surprise, since it was rare to see a human in the underground.