The characters of the AU remain the same as in the original game of Undertale.
The designs are altered as to seek residence in the visual style of the AU. And each character, besides Frisk, has 3 different styles.
1 to fit the hell section of the game, 1 to fit the heaven section of the game, and 1 to fit the purgatory section of the game.
You wake up in the deepest pit of Hell.
A failure of the Afterlife SOUL Sorting (A.S.S) has gotten you thrown there.
You must either FIGHT your way to Purgatory, or SPARE your way to Heaven.
Depending on where your Hell run got you, you then play through THAT section of the game. Be it killing all of Heaven or freeing from all of Purgatory.
Once you reach the end of your second area you move on to the third.
At the end of the third and final section, you have nowhere to go. You are faced with the concequences of your actions throughout the three areas.
The save points are explained as "SOUL Resurection Points" where your SOUL re-manifests. Because you can't die twice.