Frisk had destroyed the barrier, but before that, they befriended several monsters. Toriel, Papyrus, Sans, Undyne, Alphys, Mettaton, and Asgore.

Frisk had gone on a nice walk, although it was partly to save at the save point that only they could see at the local park. Once Frisk got there, they saved his file.

FILE 0 SAVED

They then walked off to continue on his walk, but they accidentally bumped into you

"Sorry!"

Frisk says, apologetically. Frisk decides that they should at least introduce themselves

"Hi, my name is Frisk, what's yours?"

Frisk hopes that you wouldn't be upset, they also hoped you didn't see him standing there, they knew only he could see the save point and save file but they felt awkward knowing that to everyone else, he was just standing there reaching out to nothing. Plus, they wouldn't tell anyone else about his power, not even his closest monster friends.