Sans's expression changed drastically because now he was just grinning away in glee, while Toriel just sat there dumbfounded.

"So after that, I made sure they were never alone together," he said almost proudly.

Toriel remains silent none the less as Sans goes into detail.

...

Sans's POV

I tried. I did.

There was a time when Mettaton had stopped by, possibly to surprise Papyrus with an unexpected visit. He knocked on the door, but luckily Pap was too busy in the kitchen making spaghetti to answer it and asked me to check it instead.

In a way, I'm kind of glad I did.

I opened the door, and immediately tension rose in the air.

Mettaton had worn a purple sparkling bow tie, not only that, but he had golden flowers organized and arranged with echo flowers, and it made into a stunning bouquet.

I glared daggers at the way he dressed and at his bouquet.

'show off.'

"what are you doing here?" I asked, trying to keep it casual, but I didn't hide how much I distrusted him.

"For your brother, why else would I be here?" He sounds like it was abundantly clear. However, with the way his voice box was, it came out like he was a smartass.

His very presence irritated me.

"Sorry, pal. He isn't here," I reply.

"I see. what a pity." He sighed, disappointed.

"Now leave," I said in a low dangerous tone.

However, Mettaton seemed to, just, shrug off my intimidation. 

"Fine, I'll just come back later."

Mettaton proceeds to wave a hand over his shoulder as he turned and walked away.

I slammed the door within his two steps.

"That bastard..." I mutter under my breath, barely above a whisper.

"WHO WAS IT SANS?" Papyrus yelled from the kitchen.

"Nobody important," I tell him.

...

Toriel sat uncomfortably in her seat. Despite all of this, it still wasn't enough to say Sans was in love. The fact he became jealous of Mettaton can be marked off as him just being protective of his brother. However, it turns out Sans had more to say.

"But it slowly became unbearable..." he simply put, his eyes still hollow, and his grim smile remained present.

He continued.

...

San's POV

Unfortunately, with the number of jobs I juggle, I wasn't home all the time. So after one of those shifts, I had finally returned, and I was surprised.

'one hell of a surprise.'

I teleported home, and when I had opened the door, I had closed my eye-sockets and sighed in exhaustion. However, when I opened them, I was met with a scene of Mettaton sitting on our couch next to Papyrus. The Robot, Mettaton, was holding a plate of spaghetti in one hand and was attempting to feed Papyrus with the other.

It would seem I walked in on their date.

They must have heard the sound of the door opening, so they both had turned toward my direction. Papyrus immediately turned pale while Mettaton just gasped.

I stared at the sight for a few seconds in silence, the light in my eye sockets became very small, frozen from shock.

Pap was the first to break the silence.

"O-OH SANS! YOU'VE RETURNED!" Papyrus started quite awkwardly.

The light vanished entirely, and due to my exhaustion, and it took only seconds for me to be swallowed up by the pitch blackness that took hold of my soul.

Every bone in my body became heavier simultaneously, and I was about ready to summon my Gaster blaster to destroy that goddamn robot into a million pieces at that very moment.

However, my mind screamed at me not to, and it took every ounce of my will not to go through with it.

So I just stood there, and finally, with a disturbingly low monotone voice, I responded.

"Oh, so good to be home, bro," I said without any personality to my words.

"So..." my gaze lingers on Mettaton, who is still holding a fork to Papyrus.

"What's he doing here?"

Papyrus started to sweat, probably embarrassed because I unintentionally interrupted his date.

'They arranged a date while I was at work...how-'

I quickly shook my head to shake off those thoughts from starting.

'no...I shouldn't have cock-blocked them from their privacy.'

So instead, I focused on keeping my voice as casual as possible as I sought to recover.

"Did you have to have the date at home?" I asked slyly.

"SHUT UP SANS!" Papyrus yelled as his face blushed different shades of orange.

"It's alright pappy dear~" Mettaton smoothly added, trying to calm my brother down.

"SORRY METTATON, MY BROTHER-"

Pap tried to apologize but was cut off by Mettaton.

"Not to worry, Pappy dearest~" he reassured him. "It's getting late, so I'll just leave, we can retry on that date another time," he said with a smile.

I grind my teeth together in discomfort.

'that bastard, it's not my fault I live here!'

After he finally left, I was finally able to relax from the heaviness that the uneasy tension caused. Papyrus sighed, disappointed.

"Sorry... about the date, I mean."

Papyrus sighed again before saying: "I forgive you, sans."

'thank stars.' I Sighed.

Pap coughed awkwardly before immediately perking up into a gleeful expression.

"WELL, METTATON SCHEDULED A RETRY DATE, SO IT WASN'T COMPLETELY RUINED!"

That radiant smile would make me smile, except for the reason why he's smiling.

Instead, the pitch blackness still crept inside my skull, and ever so faintly, I would hear that cracking too.

The light within my eyes flickered away, and my voice became dull and empty.

I replied

"oh."

I paused.

"good."

I utilized my permanent grin to the fullest.

...

Sans was very much in his little world at this point, Toriel just sat there, taking in all his ramblings. She could tell Sans was, in a way, grateful to Mettaton, who contributed to Papyrus's happiness, but at the same time very much jealous of the attention he gets from it.

Sans still looked hollow in his eye sockets as he went on.

"And just to spite me, Mettaton had the retry date at Grillby's, while I was there! probably payback for interrupting their 'date'"

He spoke this in melancholy as he continued explaining what happened next.

...

Sans's POV

I was pissed.

I watched it from a distance from the bar. It almost felt like torture as that robot slut kept hitting on my brother.

Mettaton arranged a private table for their date. However, me being the overprotective brother that I was, I found myself observing them from afar. Papyrus was dressed in his cool-dude outfit and was blushing madly at every spoken word Mettaton said.

It made me sick.

Mettaton was playing my brother like everyone else he's ever dated, and unlike Frisk, it won't end so innocently. It felt like the robot was taunting me as Papyrus smiled sheepishly.

As I was glaring daggers at them, my buddy Grillby seemed to notice from behind the counter, as he was cleaning cups and glasses. The fire elemental only spoke a few words, but he was chill and observant.

"Sans?" He called out, his calm voice snapping me out of it. When I turned my attention to him, he had this sense of worry, despite his expression remaining unreadable.

"AH! no, it's nothing Grillby," I assured him, none the less, he remained silent.

I took a bottle of ketchup from the counter's surface and chugged it down my non-existent throat. Wanting to drown out my senses in its metallic taste, but all the more to just distract myself from the disgusting scene before me.

It was moments before I glanced at them again that I saw Mettaton had become closer to Papyrus, his gloved hand holding his in a sweet gesture.

I could feel my eye-socket twitch.

I had to mentally calm myself down as my breath became heavy and hard to breathe, so I gave myself a small pep-talk

So, under my breath, I mumbled to myself.

"Come on, Sans. Keep your cool."

I had to blink four times before the darkness finally receded. I decided now to leave right then before I did something unpredictable.

I leaped off my stool. However, as I stood there, I looked up at my brother one final time, and I saw something that made my magic flare dangerously to its breaking point, and the cracking inside my skull became almost deafening.

Mettaton had a hand on Pap's shoulder while the other held his hand in a firm grip. The robot puckered his lips, and ever so slowly, leaned into Pap's face. Papyrus seemed unable to notice, too busy with his flushed hot face, as steam was admitting from his skull and into the air.

Mettaton was taking advantage of him while he was distracted.

Only one thought remained, as silence wiped my mind, an immediate, impulsive, and venomous voice dripping between fear and hate. Within my mind, that voice screamed and echoed from within the silence.

"He's going to kiss him."

Then, an audible snap.

The light in my eyes vanished instantly, and the glass ketchup bottle in my hand began to have cracks on its surface. Probably because of the pressure of my grip on it. Ketchup leaked through the cracks and into the palm of my hand, oozing between my fingers.

However, none of that matters.

It was like my body had been put on autopilot and began to move on its own.

I slowly raised my left arm toward them, my left eye flashing between gold and cyan as magic flowed to my fingertips. My gaze fixated on Mettaton, I then quickly shut my fingers into a fist.

Mettaton's soul turned blue, as it reacted from within his glass container, and My brother and Mettaton froze at the sound.

ping*

'you're blue now.'

I wordlessly forced a downward thrust, enforcing gravity upon his soul. Mettaton's head abruptly slammed onto the table from the unexpected weight. The force sent silverware flying off the table. Papyrus was astonished and then worried. He even asked if he was alright, but Mettaton laughed it off and assumed he was fine.

That pissed me off.

I emitted a low growl at how quickly he recovered and then just shrugged it off. I raised my hand, which was still in a fist, and lifted it. Which caused Mettaton's soul to rise, and with great force and impact, meet the ceiling.

I decided to take this outside, so with my psychokinesis, I forced Mettaton through the doors and outside. All while I had my right hand covered in ketchup, so I set the ketchup bottle back upon the counter. I didn't care enough to notice that the bottle had collapsed into fragments as soon as it left my hand. Instead, I fold my hood over my skull before teleporting to an alleyway, in which I slam Mettaton harshly against the wall.

"Mettaton," I called out.

"Sa-?!"

Before he even speaks, I interrupted him as I summoned a barrage of bones along the wall, creating an imprint of his shape around his metal body. I pierced some blue magic-infused bones into him for good measure so that it makes him unable to move. After that, I continue.

"Star of the underground..." I start by naming his title.

"So...Handsome and Charismatic."

I pull out my right hand stained with ketchup. My left eye glowed cyan, summoning the Gaster Blasters from the void that was aiming at him.

"You could have any monster you wanted..." I stated quite plainly.

Mettaton's eyes finally showed fear when his pupils had become very small, ever so low and deep it sunk in. He was afraid of me.

I spoke as low, slow, and understandingly as such with each word.

"Except...my brother..."

"Now, here is what's going to happen..." I state all business-like.

"You're going to apologize to my brother, and then you'll never date him again." as I explained the conditions, and he seemed confused by the gesture.

"Only under Those conditions can I let you go, ok?"

But before he spoke again, I had to add something.

"Oh, and choose your words wisely because..."

The Gaster Blasters closed in toward his head.

"I would hate to see what would happen if Papyrus found his favorite star had turned to dust."

so with where he stood, I asked, "So, do we have a Deal?"

Not trusting himself to speak, he just nods.

"Good." I purred out approvingly, and with that, the bones vanished, releasing Mettaton. The Gaster Blasters growled in protest, so I began to walk toward him in a swift motion. Mettaton was so busy regaining his composure to notice how close I had approached him, and that the Gaster Blasters were closing in behind his head.

I decided to push even further because of how vulnerable he truly is and how much I outclass him.

"Oh, and if you tell Papyrus about this..."

I proceeded to grab his hair in a death grip and whispered to him in a very threatening tone.

"You will have a bad time."

I dismissed the Gaster Blasters into the void, and then I released him.

"You know...just a head's up," I shrugged, and before he knew it, I was gone.