(Well then, with my other story doing so well... I thought that I might try this one too. I have a WD Gaster story made, and a Jevil one... but I decided to do this instead lol. Probably because I wanted to draw Horror ;PAnyways, this story will be receiving slower updates in comparison to my Nightmare x Reader. As a result, this one will get a new chapter once or twice a week at the minimum until I finish the other book. Gaster's use of Wingdings is in italics for easier reading. Enjoy~)

---No one's Pov---

After a long fifteen years, she finally managed to do it. With a single blood stained hand, her knife ended the existence of her family's killer. He looked up with an almost pleading expression, begging for her to do nothing... but why would she waste this opportunity?

He asked for her forgiveness, claiming to have a wife and children... but she didn't care. She was a killer, and he carved his own path once he decided to kill her family.

After all, she has been trying to do this for a long time. Her LV was very high from the effort that she put into this task... everything was completed. Well... Almost everything.

Once she returned back to her hideout near mount Ebott, the girl threw her blade into a nearby wall. It struck against the photo of the person that she finally killed, always precise with very little flaws. This would be the last time that she'd ever take anyone's life... from now on, she decided to become a pacifist.

The shadows around her twisted to reveal the face of her partner in crime, he was a deformed skeleton that communicated in hands. Or more specifically, Wingdings. WD Gaster, the name of this creature, took care of her after the death of her family... he taught her everything about his creations, the structure of souls and overall, anything to do with science.

Ever since she was born he would appear to her, and her parents would assume that he was just an imaginary friend... but it turned out that he was actually real. He just lost his existence to this world after falling into his own creation. Gaster used to roam the Underground, watching the progress of the monsters until the Core's failure.

When the Core went inactive, it forced an imbalance in magical energy, causing him to no longer hold a stable connection with the area. Fortunately, he was able to find this human that could see him, thanks to her unique soul.

While Gaster claimed that the Soul seemed like an odd mix of determination and integrity due to it's lilac color, he could only finalize the trait as 'Perception.'

That being said, the fact that she could see him at least made some form of sense. Regardless, he led her through the horror of her parent's death by telling her where the intruder was while also managing to soothe the girl until the issue was resolved. Because for the old Royal Scientist, this was going to be an opportune investment.

---(Y/n)'s Pov---

I finally managed to return to my little hideout, satisfied with the kills that I managed to make. My soul didn't feel the slightest amount of guilt for the deaths that I caused, it only responded with relief. Finally, I wouldn't have to live with this unbareable regret. "Doctor Gaster."

Once the words left me, I received a curt nod from the one that made everything possible. He gave me the necessary tools to pull off this whole thing...

The former Royal Scientist taught me how to fight, outsmart and kill my targets. He led me to the killers of my family after we made an important deal. And if anything, I took pride in being a person of integrity. "You held up your end of the deal. So now, I will hold up mine."

To maintain our relationship, I had been gathering the tools that I needed on the surface to complete our deal. He has been teaching me what he knew about science and monster biology... everything to prepare me for this. I was going to repair the Core.

"As you told me, the rest of the parts can be found in the Underground, scattered." He offered a nod to me as I moved towards the papers on my desk. They were all written out in hands, wingdings.

Naturally since Gaster communicated in that language, he taught me how to read and write in it... the downfall? I couldn't read or write in English. All of my notes were written in his dialect... which was fine since no one else would be able to read it.

Of course since I have been stealing all of these parts for the project, the authorities were after me. I only had maybe a few more hours until they'd be searching this whole mountain side. "Alright. Time to pack, what will I need?"

Once the question left me, he started to communicate with a swift movement of his hands and words, the sound that he made was generally impossible to understand... unless you grew up learning it. "Pack yourself some food. I am unsure of the situation since the Core's power failure. It is unlikely that there will be a lot of sustenance."

"Check." And in an instant, my bag was filled with chocolate bars, cookies and some candy. This caused the skeleton to give me a look that showed his displeasure. He was still smiling, but it was more of a... 'really.' type of expression.

"Bring that bottle of ketchup. There will be no greater ally to you in all of the Underground."

"Chec- Wait, what?" Now it was my turn to give him the absurd look. He seemed to be deeply in thought for a moment. "Uh... isn't that a little-"

"You're absolutely correct, my young assistant." He suddenly said, causing me to sigh in relief. "That wont be enough. Bring two bottles."

By this point, I wasn't going to try and argue it. Gaster always had a method to his madness. So with that, I took bottles from my food stash and put them away. If I could have my sweets, he could have me bring ketchup I suppose.

"Do you have the prototype with you? And be sure to take that map that we designed." he added. Putting the map in my bag, I could remember the time that we tried to create it. Gaster attempted to inform me about the Underground through the drawing of a map, reliant solely on his memory.

I've charted everything through our constant sessions, but I still haven't been there personally... I am aware of how the ruins work, so I will manage to get through that with little effort. Snowdin Forest has a strong layout as well, and Waterfall's design is practically drilled into my head. But still, I might struggle without a decent guide...

Because when I fall down there, I'll be on my own. Gaster cannot follow me since the Core managed to destroy whatever stabilization there was. That is why I have taken every precaution possible when learning about the Underground.

Snatching the prototype from the desk along with all of my notes, I stuffed the papers in my bag but put the device in my pocket. I wanted it to be as close to me as possible... It was very small and had several wires connecting to a single orange orb that was filled with a liquid. Let's just say that the substance is what originally made the investigators come after me.

After killing my targets during my little hunt... I was extracting the essence of human souls to substitute as a type of fuel. The Core would need an extra amount of strength to run, which was reliant on something powerful enough to give it life. And trust me when I say that the procedure was very painful since they were alive during the extraction.

I also had to get my hands on a very... high grade fuel that a powerful group of people hid very well. So basically, the equivalence of this prototype exploding would have enough strength to mimic that of a massive explosion the size of a city. Good thing that it wasn't built to somehow detonate.

"(Y/n). Dress appropriately, make sure that you bring your coat and scarf." Oh right... that. I almost forgot that Snowdin was absolutely freezing for whatever reason... something to do with the magical atmosphere that the barrier encouraged and that the Core once regulated.

Quickly I slipped on a very long coat that covered the top of my torso, but extended down in the form of a cape to protect my backside from the wind. It almost looked like Gaster's lab coat and was a black color to keep me hidden during these dark nights. Additionally, I wrapped my silky black scarf around my neck... it was better for fashion, but still protected me from the cold.

Of course it was currently summer, so I was overheating with this on...

"Water bottle and your grapple." Aha... what would I do without him? He was always so helpful when it came to reminding me. Once I had everything together, I swung my bag over my shoulders and left the hideout. It would be raided within the hour, no doubt.

I didn't intend on returning to the surface any time soon since I was very much wanted by the authorities... but regardless to that, Gaster raising me has always taught me one important thing in life. Always keep your promises.

He kept his, and I would be keeping mine. That is all that I cared about. I wasn't interested in the wellbeing of the monsters, or if my actions would make their lives better... my promise was the priority. "Remember the codes?"

"Yes." I said with a nod.

"Remember the names?" He continued.

"Of course."

"Do not forget the pieces... without them, that prototype will be useless." Gaster amended as we came up upon the hole that led into the Underground. There were many vines that could easily trip someone that was unsuspecting of the fall. Unlike the unfortunate people that fell, I had a grapple ready.

Swinging it around while looking for a decent ledge to attach it to, Gaster had one more thing to say to me. A piece of advice that I didn't avoid in the slightest. "You have a lot of experience with judgment, so allow me to say this before we part ways. If something seems out of character from what we discussed, become discreet. Do not place your faith in something that you do not trust. What you perceive as dangerous and trustworthy is entirely up to you."

"Thank you for everything, Gaster. I will do this for you, do not worry about my ability to succeed in this." and with that said, my grapple was successfully hooked on a vine and I was diving down into the pit of darkness. The back of my coat was fluttering behind me as I fell down, showing that I was falling at a rather fast rate.

Once I ran out of rope, my decent was slowed. The vine that I had my hook attached to snapped, leaving me to fall onto a bed of golden flowers.

Thankfully I didn't drop from a painful height, actually... this bed of flowers managed to break my fall just like Gaster claimed. Mind you, I didn't take the full brunt of the fall.

As I got up, my cold (E/c) looked around the room in interest, I was trying to examine my location to the fullest. Using my hands to straighten out my long coat, I then proceeded to wrap my grapple around my waist. The coat easily hid it's very existence, allowing me to hide the evidence of how I truly got down here.

The air didn't seem... normal.

Taking a few cautious steps into the next room, I saw something in the center. A flower, just like I was told about. He would claim that his name was Flowey... but I knew his true identity. "Good evening."

In an instant, the flower's head snapped up to stare at me with a single large eye- now that was different. This didn't look like Flowey... "Uh... Howdy."

He didn't match the deathly vibes that I've been getting down here... perhaps it would be alright to speak with him for now. "Looks like you fell by accident, huh?"

I offered him my neutral look of disinterest. He seemed taken aback from the face that I made. "So um... I guess that you'd be the dumb, headstrong type. Right?"

No. I definitely was not that type.

"What, nothing to say at a time like this? A flower is talking to you, and you're not even flinching." By now I think that I had enough. I could tell from his childlike sarcasm and placement that he was who I was looking for. So I said the only logical thing that I could think of... because I felt very insulted by him.

"Please do not suggest that my intelligence measures down to the equivalence of a rock."

Welcome to the Underground I guess... Time to keep a promise.