You was a human that fell down mount ebott you made it out of the ruins and was walking down snowdins forest when you heard a girl around about your age speak to you

Aliza: y.your a.another human

You:uhh yeah

Aliza: you can't let the monsters see you if they do they will try to eat you that's how I lost my right arm...two skeleton brothers call sans and papyrus found me and took me to snowdin village and brought me to grillby's where the monsters put me on a BBQ I got away tho