It is not explicitly stated that Frisk ever fell into the ruins, it’s just assumed. At the beginning, you name a fallen child, which you later find out you are naming the first human, which you come in contact with later on, depending on what route you take. During one playthrough, Asgore asks if you’d like some tea, being quite polite but knowing that he has to fight you as if you were human to gain your soul. During a different playthrough, he asks what kind of monster you are and that he doesn’t recognize the type, but this can also be Chara seeping back into this reality, since it is the “bad kid” route, or g