Undertale meets Shield

Were going on a trip in our favorite quin jet

It was finally morning and Nat was walking through the halls in search for a certain monster. She stopped and turned to her left to see a powered down Mettaton. She turned to them fully before walking to the cell door."Are you still alive?" Nat asked with a smirk, Mettaton looked at her and scoffed. "Not for much longer." They said, Nat raised her brow. "I heard you're going to Hotlands in search of a certain monster. Meaning nothing stopping Mr. Angry eye patch back there from ripping me to shreds." They said. "I know. He told me he wouldn't do anything like that." She said and looked at the small box she had in her hand. Mettaton raised a brow.

"But you don't believe him?" They asked, Nat shook her head. "If I've known fury for as long as I have, I know he's good at lying to even the people he trusts the most. I've got one thing that will help me take care of what's happening." She explained quietly. "But you need an actual spy. One he won't expect to find a wire on one made completely from parts." Mettaton went on as they stood up and walked over to Nat. "Yeah, do you have an area that you think he won't look into?" She asked, Mettaton smiled.

"Yes, and it's not gross." They said, Nat smirked and passed the box through the food slot and Mettaton grabbed it in tie before it dropped. "A little hint darling..." Mettaton said before Nat left. "Some monsters were stuck in their old ways and didn't leave. While some of us wanted to leave, others were too afraid." They said, Nat looked at the ground. "Do you think there's anything you can tell me?" She asked and turned back to Mettaton. "When Gaster fell into the core, I was in the middle of being created. He helped make me but since I'm not a real monster." They paused and touched the small compartment that held their soul. "My memories weren't fully wiped. Whatever he did to his boys, he did start to care for them in the end." Mettaton said and opened the bow to see an ear piece.

"He was the last of his kind after the war and was getting desperate. He only wanted a family but he wanted answers and saw the boys as a way to answer them. In a funny way, I kinda understand how Sans felt." They said and looked up at Nat. "I'll try to hurry back. Just hang in there." Nat said and put her hand on the screen stopping them both from touching. Mettaton smiled and put their hand on the same spot. Nat smiled and walked away. She passed through the halls of the monsters and was unaware the monsters were watching her with hopeful eyes. Even though she betrayed them, why were they hanging onto hope? The hope that she'd free them all. She waited until she was at the end of the hall to ball up her hands and look at the ground. Is this how Frisk felt? She thought to herself. If so, how did they win the battle? She thought to herself again.

"Nat?" Someone asked, she yelped and looked to see Stark with his arms folded. "We should get going. The faster we leave the sooner we can come back and do tests." He said, she bawled her fist and glared at him. "Let's just get this over with." She said and pushed him out of the way with a growl. They made their ways to the quin jets and flew off.

Time skip...

They finally made it to the safe house that still seemed to be there. Jarvis turned on the cloak as they landed and the back door opened. "I should have some extra gear in the safe house." She said and walked inside, but when she touched the door, it fell into the house to reveal it was trashed with everything destroyed. "Correction, I had some extra gear." She said and sighed. "Jeez, whoever did this, wasn't happy." Stark said, Nat sighed and walked in. It was silent until they heard a crash. Everyone aimed their weapons as Nat walked to the noise.

"AH!" She yelled and aimed her baton as she grabbed a blanket to reveal Frisk. "Frisk? what are you doing here?" She asked, Frisk growled and grabbed a knife next to them. "LOOK OUT!" Steve yelled as Frisk jumped and sliced at Nat with their eyes opened to reveal them to be ruby red. "How could you! They just wanted to live a peaceful life!" They yelled and tried slicing more as Nat dodged, Thor grabbed Frisk and tried to pull away the knife. "I TRIED SO HARD TO GIVE THEM WHAT THEY EARNED!" They yelled and tried to stab Thor's hand. Thor dropped Frisk as tears went down their eyes.

"Calm down Frisk." Steve said, Frisk gripped the knife and threw it at Steve's head but it missed luckily. "Do you know how many times I died trying to prove that violence isn't the answer?" Frisk yelled and balled their fist. They started glitching out while crying. "Do you know how mean people are in the orphanage?" They yelled at them. Banner walked over to Frisk. "You need to calm down, take it from the guy who turns into a giant green guy when he's mad." Banner said, Frisk laughed as their form changed in a goat monster appearance.

"I'm not mad, I'm, I'm." She stopped and punched the ground. "You're very bad at lying, other me." Another voice said, the avengers yelled to see a colorless black eyed version of Frisk. Frisk looked up at Core!Frisk. "You're afraid of turning into me if the other monsters aren't around." They said. "Who are you?" Clint asked, Core looked up at Clint. They walked over to grab a chair and pushed it over to Clint, climbed onto it and looked at him. "Uh, you didn't answer m-" He was cut off when Core punched him hard in the face, causing him to fall. "That was for Ink. He's done so much for your world but monsters will be monsters I guess." They said.