A kind author requested this idea from me.
I hope they will not find it too sloppy...

Some guidelines for this particular story:

"Hi, this is Chara" - normal dialogue for Chara and the other monsters.

"Hello there, this is Gaster" - Gaster's speech in some instances.

*This surely is interesting... - Gaster's narration in some instances.

"...you'll never remember" - marks flashbacks.

"This is my mind." - Internal thoughts.

Prologue
Strange Changes

"...you'll never remember anything."

That was all Chara could hear, before being impaled by three bones erupting from the Judgement Hall's shining marble floor. Sans watched with satisfaction as the child's SOUL broke in two, and their lifeless body dropping with a loud thud on the crater left behind by the skeleton's final attack. He smiled, exhausted after the long battle, and decided to give a call to Asgore, let him know that the genocidal maniac has been stopped, until-

The Hall was starting to fade from existence, as if a RESET has been pulled up by the child, yet Sans didn't understand what was going on and kept watching as darkness slowly engulfed everything. His left eye flared cyan, steady as he was whenever a RESET was called forth, yet one thing plagued the somewhat lazy skeleton - if they knew that defeat was all that waited for them in here, - why bother resetting?

Yet this particular RESET wasn't like the others.

The darkness suddenly started glitching, and Sans could notice a few rays of light piercing through the darkness, until he was engulfed and disappeared.

After the strange event that made Sans disappear in a flick of light, a strange human form could be seen crouching over the lifeless body of Chara's. The mysterious man placed a hand over it, a faint light basking the body and restoring the SOUL that was once broken, the child opening his eyes wide. They put up their hands in defense as the man's face was smiling at them in a strange, almost terrifying way.

"Who..." Chara spoke with a small voice, "Who are you? And why did you save me?"

"My name doesn't matter right now." the man said, as his smile vanished. "What matters is that we work together to save this timeline."

"What? Are you insane for some reason?" the striped-shirt human furrowed their brows in anger, as they dusted their clothes. "Do you even know what I've-?"

"Yes. You are willing to commit Genocide." Gaster said. "But you must learn to be a Pacifist, or otherwise..." the tall scientist sighed, "Dimensions will be shattered in an endless cycle."

"Give me a good reason why should I care." Chara huffed in annoyance, while the scientist looked at them with hollow eyes.

"Because you have a hint of good inside you," Gaster said. "No matter how much you deny it, you wish to save everyone somehow."

Chara closed their eyes, not willing to believe what the mysterious man just told them, yet he was right on one thing - they wanted to save everyone - but their hatred was too much to bear, that it took control and forced them to commit genocide, over and over. "This guy isn't kidding at all when he says I want to do something right," the child mentally sighed, "but he's wrong if he thinks I'm going to be selfless."

"That expression..." the scientist said as he turned his head towards the child, "You believe that you can't ever be a selfless person, is it not? We'll see."

"What's that supposed to mean?" Chara raised an eyebrow in questioning, their arms folded.

"I shall impart my essence with you. I'll be guiding your every step..." Gaster said as he placed his hands over the child's shoulders. "...And I'll make sure you don't fall back into committing genocide again."

"Again, why do you care about thi-" Chara said, before Gaster silenced them.

"Because you are a lost soul, just like myself." the scientist said. "I want to free myself from this endless abyss that is the Void. And only the Pacifist Route can help me achieve this."

"Fine," Chara nodded as they dropped their stance. "I'll let you guide my way, though don't pull any tricks on me."

"The timeline will decide if it will pull tricks on you or not." Gaster said. "Now...let's commence the transfer."

Black tendrils were engulfing Chara for a moment, the latter standing still during the whole process, wondering what could happen worse than that. A part of Gaster's essence attached itself to Chara's soul, in the form of a purple patch, and then the scientist's tendrils released their grip on the small child.

"Very well. Now we wait for the timeline to adjust to these changes..." the stranger's voice resounded in Chara's mind, "...And we'll see where we end up."