Frisk used to be Kid (The MC for Kindergarten) but eventually ran away from home and fell into the underground, eventually after a natural run or just leaving at the end of the pacifist run they wind up on the streets and eventually start living inside of the Pizza Plex changing their name to Greggory.
I figured that they all are children who wear striped shirts, all different ages in my mind with Kid being the youngest and Greggory the oldest, and they all have the power to save and reset.
After Greggory grows up he joins the circus since it helped with his homesickness for both the monsters and Freddy, before he gets wrapped up in the mafia becoming Skull.
I forget what memes I used, if you know please let me know so I can credit the right people.