[Disclaimer: I do not own Familiar of Zero, nor do I own Undertale.]
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It was official. She was stuck here... wherever here was, anyway.
Her name was Louise Valliere, and recently, she had failed the Familiar Summoning, somehow finding herself trapped in this strange, black space. Odd thing was, she felt a lot less powerful than before.
"Why can't I just go back?!" was what she yelled to herself and, coincidentally, the empty dimension that she currently occupied.
Louise knew it was empty, so she wasn't really concerned about what she did or said. After all, there was nobody around to here her.
So it surprised her that a response came.
"It is not that simple."
She quickly turned around, and saw him.
The first thing that could be described of him was that he was not human, instead looking more like a cartoonish skeleton. The only things visible about him were his head and hands, the rest of his body being wrapped in shadows. Two scars ran across his face, one going from his right eye up, the other going from his left eye down. Both of his bony hands had large holes in them.
Louise felt fear, and the odd skeleton seemed to pick up on that.
"Do not feel afraid, child. I will explain." And explain he did.
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It took a while and a lot of repeated explanations of multiple things about what he was and where they were, but Louise finally got it. It would understandably take a while, since many of the concepts of the skeleton's story—W. D. Gaster's story—were alien to here, but she finally understood.
"So, let me see if I got this right. You were once a royal scientist in a sealed-off area below a mountain, eventually called Mount Ebott, working for a king of a kingdom of multiple kinds of monsters. You were called on to create the CORE, find a way to break the barrier keeping the population of monsters trapped in the Underground, and study the trait-slash-energy known as Determination. However, you accidentally fell into your own creation, causing you to scatter across space and time, reducing your soul to one-eighth of itself, and also causing everyone except your older son Sans to lose their memories of you. You then have been stuck in this place, the Void, ever since." was what she summarized. Then, turning to Gaster, she asked him, "Am I correct so far?"
"Yes, you are. However, I haven't exactly finished explaining everything about my world."
"Really?" Louise was surprised. There was more that she hadn't heard of yet?
Gaster nodded. "But, before I tell you, I believe I have a way of determining why you feel weak." Taking a pause to let that information sink into Louise, he then continued with, "Would you like to try it?"
Louise, once she realized that Gaster wasn't lying, nodded without hesitation.
"Well then... here we go."
Suddenly feeling a slight pulling sensation on her chest, she looked down just in time to see something come out of her chest. She and Gaster both stared at the cartoonish red heart floating in front of her.
Or the one-eighth that was there, anyway.
"That... explains it..."
Louise turned toward Gaster when he said that, saying, "What do you mean?"
"It appears that your soul has suffered the same fate that mine had—it has broken." Said Gaster as he put Louise's soul back into her body
"Oh..." Silence, and then, "...Why is it red?"
"Well, you seem to once have had a full soul filled with Determination."
"What does that mean?"
"Well, I'll have to explain a bit to you. You see, there are seven types of human souls, each one representing a different trait: Patience, Bravery, Integrity, Perseverance, Kindness, Justice, and Determination. All of these traits are special in their own right, however, Determination is the most... unique. While they are helpful traits, Determination allows the user to, put simply, control time."
Louise practically felt her jaw drop at the information.
"However, even if you have the power of Determination with you, I don't believe that either of us can really get out of the Void, which is why I am proposing a solution." Then, what he did next surprised Louise a lot more than before.
Gaster took out his own shattered white soul, and promptly gave it to her.
"Wh... why...?"
"It is the best chance we have at getting out of here. Hurry and take it, I can already feel myself turning into dust."
True to his word, his body was, from the bottom up, slowly beginning to turn into dust. Louise realized that she had to make a choice, and fast.
And then she decided.
Her own soul popped out of her chest, and with a little effort, she grabbed Gaster's soul and placed it with her own soul, watching as the two pieces hovered next to each other. Looking back up, she saw with a saddened look, that Gaster was almost done fading, but noticed that instead of a look of fear at his demise...
On his face lay a sad smile, one filled with regrets.
And then he spoke.
"Goodbye, Louise..."
And then, finally, W. D. Gaster faded into dust.
Before Louise could mourn for him though, she felt a sharp pain coming from her soul, her face, her hands, and her mind. Fearing the worst, she turned to look at her soul... and watched in fascination as the two fragmented souls—both her's and Gaster's—merged into one.
She then rubbed her face, and realized that her soul wasn't the only thing about her that had changed. She felt her face and hands... and noticed that she had gained scars on the same places that Gaster had them. She even had empty eyesockets where her eyes once were, and yet she could still see.
However, Louise noted that it wasn't finished, as memories began pouring in like a waterfall, memories that were not her own, but Gaster's.
Soon the pain receded, and the new monster-human hybrid looked at herself, coming to the realization that she wasn't just one of them, but both of them combined.
"I'll need a new name for myself..." She thought. "Aha! I got it!"
Both Louise Valliere and W. D. Gaster were dead, and in their place, Louise Gaster was born.
"Now then, what to do..."
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[I may or may not continue this story. I kind of like the little idea I have going here, though.]