Phantom Of The Stars

We know very little of the past of the fallen humans in Undertale. Even Frisk, the main character of the game, has only little confirmed information about them. 

Whilst no official answer has been given regarding their past, there are however a number of clues in the game that seem to point to a certain direction.

Frisk exists as their own individual outside of the player, and in those instances where Frisk acts or talks on their own free will, they leave hints concerning their own personality, likes, dislikes, fears, and what appears to be hints of their past.

Here is something that Frisk asks Gerson on their own after a true pacifist ending.

How far will you go in the name of thy justice?