About the AU
The only truly changed character is Papyrus, though Sans, to an extent, has changed as well, as a reaction to the unfortunate condition of his brother.
The AU's story goes along as so:
The human, after growing bored and having lived many lives (be them pacifistic, genocidal, or anywhere in between) decided to try a little experiment. They were aware of Sans' continued docile nature in the event where not EVERYONE had been killed, even if they had only skipped over two or three monsters. Most notable to them was the fact that he didn't attack them when Papyrus was killed.
They sought to change this.
So they set up a plan.
Get through the Ruins. Get to Snowdin. Fight Papyrus. Kill him. Reset.
Normally, this wouldn't change anything. He'd been killed countless times before, so why would this matter? However, Papyrus' data just couldn't handle the constant shifting and changing between life and death. After all, nature isn't meant to bring back something that is supposed to be dead.
Over time, the data began to corrupt and twist, both changing Papyrus' appearance, and causing his physical and magical capabilities to be severely downgraded. (Learn more about Papyrus here.)