Oni meets Onii-chan

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"It hurts"

I kept telling him... But he never listens.

"Stop"

But he never does

"Eri-chan, you are the key to cleaning this filthy world! Why do you keep resisting"

He clenched his fists in rage... His eyes glared at me. I already tried running away multiple times... But he always stops me

"I-i- don't wan-"

"It doesn't matter Eri, I will cure this cursed world. Even if I have to kill you"

Kill? I felt tears running down my cheeks. I tried to wipe them away but my hands were handcuffed to the bed.

I struggled with them but my arms hurt. My whole body hurts... It hurts....

"Eri, were getting close. Soon it'll be finished" He said, I saw him fiddle with a red thing in his hand...

"Just a few more tests"

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Waiting in front of the Kai Chisaki compound I awaited the perfect opportunity to sneak inside. Unfortunately, it is taking a long time due to the guard rotation. The good thing is that I had almost all the time in the world.

*Shiver*

One of the guards suddenly shivered in his spot. Making the others stare at him strangely.

"Are you alright?" He asked while putting a hand over his shoulder, seeing small sweat beads on his forehead.

"Y-Yeah I'm fine just felt like someone is staring at us" The guard scratched the back of his head as if what he said was ridiculous, when in fact they were being stared intently at by a blue-haired fellow.

'leave leave leave leave leave leave leave come on hurry up leave leave!'

"We better hurry up or the boss will get angry," the other guard said while softly pushing the other.

"Yeah... Sure... Love ya" The guard wiped the sweat off his forehead before continuing forward not before subtly checking behind his back.

"... No homo right?

"Of course"

...........

'Finally, I can continue on my covert operations' I hopped off the building and sprinted towards the fence of the compound. Taking advantage of the guard rotation I managed to sneak halfway in before any guards were there to notice me.

Now acting much more quietly I began tiptoeing towards the main entrance of the building, which was being watched by 2 guards, both seemingly exhausted as they couldn't stop yawning.

Continuing to yawn they barely hid the holstered guns they had in their belts. I'm not sure how they got guns into Japan but they're criminals so I didn't question it too much.

After waiting for a while the guards eventually opened the doors to someone who was trying to get outside, using this opportunity I sneaked past them and directly entered the building. Phase 1 is complete!

"Now where is that secret room" I massaged my chin while staring at the assortment of hallways lying in front of me. Shrugging my shoulders I began to walk randomly, I just hope I don't get lost...

Hey is that Zoro---

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Now evening I stood tiredly in front of the supposed secret door. It wasn't much of a secret if you knew about it... Anyway, after wasting enough time, I realized how annoying villain lairs could truly be

"Alright here goes nothing" Without wasting a second I move the flowerpot aside before pressing the buttons. However, I soon realized I had no idea what was the passcode...

Pressing randomly I just hoped I accidentally pressed the correct combination however it seems Lady Luck isn't willing to show me her panties

Ditching that idea I decided to go for more drastic measures, creating a black mist in my hands I pressed it against the wall and my hand began sinking into the concrete as if it wasn't even there. Like a chain reaction, the entire wall began dissolving as the secret stairway revealed itself to me.

"Let's hope nobody notices me" I muttered out loud as the chapter needed more dialogue, calmly walking down the stairs I was confronted with the familiar concrete hallways that the MHA series showed.

"This place definitely needs some decor. This is depressing" I commented while staring at the murder dungeon. Sighing once again I began to wander the hallways yet again. Fortunately, it seems I hadn't alerted anyone yet as I haven't even encountered anyone recognizable.

It seems the stealth mission is going well

...

...

...

...

"How soon can we begin mass production?" Suddenly hearing a voice I hid behind a corner, attentively listening.

"Most likely in a few months however I'd like to start creating the finished product" Two voices began conversing. Their voices were muffled due to their plague masks.

"Are you sure? We can't guarantee Eri would be able to endure the next phase" the voice replied with apparent concern.

"Worst case scenario, I rebuild her body" The other sounded completely unconcerned, he kept calmly walking. Obviously, it was Kai Chisaki. He turned the corner with his subordinate however his eyes narrowed looking at the hallway.

"Do you see it too?"

"Yeah. It's dirt" The subordinate bent down touching the small dirt clump on the ground. His gloves stained slightly brown.

"Do you think it's the intruder?" He asked while surveying the rest of the hallway, fortunately, it didn't seem like any more dirt made its way here.

"If a subordinate brought filth here I would've killed them a long time ago," Kai said while scratching his arm, he felt absolutely disgusted. Someone is spreading filth around his compound.

'huh? Intruder? They already knew about me?' meanwhile Rimuru was absolutely confused as he lay flat attached to the ceiling. His invisibility came in clutch as he continued listening to the two's conversation.

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"It's time to sleep, Eri." Kai tucked Eri into bed after a gruesome testing period. Her arms and legs were bandaged and her horn looked to be sawn off at the tip.

"Sigh.. soon" After exiting her room Kai began wandering the halls. He also had other responsibilities other than the development of the anti-quirk darts. Managing his crime syndicate was a tough job.

"Boss I have news" Before Kai could go to his office he was stopped by the ceiling as it began to talk.

Slowly melting it took the shape of a human, of course wearing a plague mask, it was of course one of his subordinates Mimic. His ability allows him to merge with inanimate objects like say... The entire compound.

"What is it... Make it fast" Kai didn't spare a glance as he continued to walk towards where his office was supposed to be, of course, Mimic followed by having the ceiling shift at the same pace.

"We have intruders," Mimic said, causing Kai to stop in his place, his golden eyes now gazing at him.

(A/N: Am I the only one who finds Kai Chisaki hot asf? Just search him on Google I swear)

"How many" Kai asked as he began turning around. He knew most of his subordinates were currently either working or outside.

"Uh... I don't know... Someone broke the door but he was... Invisible?" Mimic said while scratching his head, he assumed it was only 1 person but a mass invisibility quirk isn't very far-fetched. His boss's quirk is more scary than even All Might.

"Tsk... Get me to Chronos" Kai clicked his tongue as the environment around him suddenly shifted to the lab where he just performed experiments on Eri.

"Boss, is there anything you need?" Out of the corner of the room, Chronostasis asked. He was washing his hands after finishing organizing the lab. Pushing aside his white hair he stared at Kai as well as Mimic.

"We have intruders"

"What a bother" Chronostasis clocked his tongue in annoyance. Quickly swiping his hands and putting in gloves he went with the three to check on the situation.

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"I'm surprised they haven't found me" I muttered in amazement as I saw them scouring every nook and cranny, I would've assumed Mimic would've noticed me as I was touching his body right? He can feel what he merges into right?

Ignoring my weird thoughts I continued walking, eventually I reached what I could assume was where Eri was being kept. The hall was much more decorated both with pink paintings and children's drawing.

To not alert anyone of my 'intrusion' I teleported through the door directly in front of Eri's bed. Seeing her sleeping face made a tinge of guilt well up within me.

'What do I do about Eri?'

Should I take Rewind and go? That would most likely require me to kill Eri... And killing toddlers is a nono.

Should I adopt her? I began scratching my chin thinking how that would go...

'huh? You want this kid to be your sister? People are not like pets... You can't just pick them up off the road'

'adopt?!?! A-ar-arent you getting ahead of-of yourself? We haven't even---'

Just imagining my Mom or Jiro's reaction gave me a clue as to how that would work. My options were most likely.

1) kill Eri and take Rewind

2) secretly adopt Eri

3)Leave Eri here and let her get rescued by Midoriya.

"Ugh, why is this so hard" I roughly scratched the back of my head, I hated taking a backseat but it's currently the best option. I could get [Overhaul] once Shigaraki gets rid of Kai's hands and leaves him to die.

"W-whos there! I heard you!" I suddenly opened my eyes to see Eri staring at me with shaking eyes. But because I was invisible her eyes darted around the room.

However, I decided not to answer her as I stood in silence as she hugged her teddy bear for strength. She slowly sat up from her bed as her horn glowed a subtle yellow.

"I definitely h-heard you! Don't pretend!" Eri shouted with determination, clutching her bandaged arms. Small tears began forming in her eyes

"Ugh, this is depressing" Not able to cope with the sight of Eri crying I stopped hiding. Of course, staying in the dark ess so she couldn't see my face...

However, my glowing crimson eyes might've scared her a bit...

"Uha! I-im not afraid of your Uncle!" Eri shouted again clutching her teddy bear even tighter.

"Ugh--- uncle... I'm only 15 please" I held onto my shattered heart as Eri called me an old man...

"Uh... I'm sorry umm Onii-chan?" Eri tilted her head in confusion, for some reason she felt comfortable around that mysterious stranger. Unlike Kai who always made her feel scared.

"Woah woah woah! Don't get ahead of yourself!" Quickly swinging my hands I denied the term 'Onii-chan'. It was a trap!

"So... What should I call you Onii--" Eri quickly covered her mouth before staring at the shadow in her room. Sighing in relief seeing it not being mad.

"Just call me Rimuru"

"Alright Rimuru-nii"

"Stop... Ugh, I guess that's fine" I sighed in surrender. It honestly felt pretty good to get the mythical 'Nii' honorific. Most of the time I'd be called a 'Chan' by people who were clearly younger than me!

However, having 2 canonical ages does complicate things...

"So why are you here Rimuru-nii?" Eri asked while slowly letting go of the teddy bear as she didn't feel danger anymore.

"Would you believe me if I said I got lost?"

"No," Eri said deadpan, while she was young she still had common sense. They were metered underground inside a labyrinth of the Yakuza.

"Let's say I'm here to comfort you until you get rescued!"

"I'm getting rescued...?.." all of a sudden Eri froze in place. Her eyes began beading with tears, the silence was only broken by the sounds of sniffles.

I was wondering what that was about... She feels completely different from canon...

She used to be a scardey cat...

"Waaaaaaaa"

'damn... I remember why I don't like kids now'.

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A/N: anyway... Haven't uploaded in a while... I got like really sick and couldn't write

not to worry this fanfic isn't being dropped (just yet) I still have some stupid stories to write with Rimuru's shenanigans