You had fallen into the Underground with another girl and were greeted by Papyrus and his brother Sans. The both of you stayed for two or three days. Then you and your friend were asked to choose between a companion. Your friend chose Sans, which left you with Papyrus, and he treated you to dinner to celebrate and made an announcement to the restaurant to make a toast.

Papyrus raises his glass "I, the great Papyrus, promise to be the best companion possible!"