"Talk"
'Thought'
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"Hello, how are you, so, my name is Koro-Sensei and I'm the one who blew up the moon~"
A smiling yellow emoji octopus, wearing a teacher uniform, who also goes by the name of Koro-Sensei, stood before an entire classroom worth of children and said children came in all shapes and sizes, ranging from normal looking humans body with minor changes to very extreme and unnatural designs

"... Huh?"
The entire classroom of students just suddenly realized and process what they just heard, and they couldn't help but wear a very confused and dumbfounded expression

"Next year I plan to do the same to planet Earth, but never mind that now, I'm gonna be your new teacher, isn't that exciting?~"
Koro-Sensei just smiles even more happily, despite saying something very terrifying to the entire class, as everyone just continues to comprehend what they just heard, another person seems to be standing next to the seemingly villainous creature, he just couldn't help but let out a very exhausted sigh

"And I am Shota Aizawa, his co-teacher and… Unfortunately, he is speaking the truth, in wanting to destroy our entire world…"
The tired-looking man, who is now known as Aizawa, confirms the legitimacy of what the creature had said, which did not give anyone any sense of comfort or relief

"And because this creature is such a big threat to the entire world, it is now your job to kill him."
And what Aizawa said next definitely gave everyone some very mixed feelings about the situation they were just told and given

"K-Kill him?..."
One of the students, specifically the one with broccoli-green hair who is named Izuku Midoriya, couldn't help but feel beyond uneasy at the thought of taking another life, even if it was from someone who wanted to destroy the entire world, a feeling shared by most

"Consider it as your first and final test of becoming a true Hero~"
Koro-Sensei, even though he was still smiling, suddenly gave off an aura and spoke in a tone that expressed something less, 'Joyful', in nature

"One day, you will face a Villain who deserves to die, and you will be one who would need to deliver the final blow, so why not start by killing someone who plans to destroy your entire world…"
What Koro-Sensei said struck a chord with everyone as if finally understanding the heaviness of their choices of becoming a Hero

"Of Course! I don't think any of you have what it takes to kill me in the first place, so go ahead and try you noobs, Nurufufufufu!~"
Though after a moment of seriousness, it went back to joyful silliness, as Koro-Sensei's face had some green stripes on them as he then started mocking and teasing everyone in the entire classroom, much to Shota's annoyance

"Of course, we Pro Heroes will also attempt to kill him as well, but it is your job to keep on trying to do so since he will be here teaching you all most of the time."
Aizawa continues to explain to the students more about their situations and what they need to do, which they somewhat come to terms with, while most aren't comfortable with it, they understood it was necessary and would do their very best in accomplishing their first ever mission in becoming Heroes

"Um, but why does he want to teach us if we're supposed to be one to… 'Kill' him?..."
Of course, while some just accepted their new reality, a few, like a certain round face girl with rosy cheeks, known as Ochaco Uraraka, were still hesitant and unsure of their predicament

"We don't know why he decided to do this, but if you do manage to kill him, then not only will all of you receive 10 billion yen each, but all of you will be recognized as the WORLD best heroes, with the one delivering the final blow  being number 1, it's only fair after all."
Even if a few were not sure about all this or were at the very least still hesitant to go first, when Aizawa mentioned the reward of actually saving the world, all of that doubt quickly washed away and was instantly replaced with either newfound interest and even a few excited expression

"OUT OF MY WAY EXTRAS! THAT NO.1 TITLE IS MINE!!!"
And a certain explosion blonde teen, who is known as Katsuki Bakugo, did not hesitate to instantly take what was openly and freely offered to him, jumping out of his seat and pouncing towards the emoji squid

"DIIIIIEEEEE!!!"
And when Katsuki got close enough to his Target Teacher, he aimed both hands at him and let out a very large enough explosion that it made Shota and the students look away, due to the bright flash and smoke

"While I do appreciate your control over holding back enough power to not destroy school property and avoid hurting anyone here."
When the smoked clear and the flash died down, the unfazed Pro Hero and shocked students were able to see a still stand and seemingly ok yellow octopus

"It, unfortunately, means that you didn't deliver a strong enough literal blow to kill me, at best all you did was destroy my outer layer of skin and mucus, which I can shed away easily~"
To prove a point, Koro-Sensei quickly removed his said damaged skin and suddenly looked good as new, even showing off his green-striped mocking smiling face

"Not only that, you were so slow in your attack that I could've dodged it at any time I wanted, even when you ignited your palms~"
Koro-Sensei then used his many tentacles to bring up a bunch of make-up, hair care supplies, and a few change of clothes

"I even had enough time to freshen you up while waiting for you to blow, see?~"
When Koro-Sensei mentioned that and showed off what he was holding, Bakugo suddenly noticed that he was wearing and looking very fresh and dapper, hair combed and gelled down, makeup to make his face more beautiful, and even had all of his fingernails painted over with very cute and detailed design, this made most of the class hold back some laughter and made the explosive blonde beyond very furious, who just simply went back to his seat

"And as you just saw, your target is not your simple run-of-the-mill yellow emoji octopus, he is simply too dangerous to be left alive, and right now, it is your job to stop this world from destroying threat and put an end to his tyranny, am I understood?"

"YES AIZAWA SIR!!!"
After listening to what Aizawa said and fully understanding the heaviness of the situation the entire classroom was in, all of the students did not hesitate to not only accept this mission they were given but do it because they were heroes and it was their duty to save the world

"Fantastic, now if you excuse me, I'm taking a nap, good night, everyone."
Aizawa then pulled and zip up some kind of yellow bean bag over his body and simply passed out at the corner of the room, much to the dumbfounded confused classroom, however, Koro-Sensei just ignored the tired teacher and faced his students in the room, pulling out a book and flipping through pages

"With all that said, please take out your 'Hero 101' textbook and flip to page 39, since today we're gonna be learning about rescuing and saving civilians, as you see when saving a person's life, you-"

'This is definitely gonna be an interesting school year…'
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Not to insult any of the teachers of UA, but they don't do much, not saying they're bad and they're not helpful at all, but, well, you know, Koro-Sensei exists now, so they're not gonna be as good in comparison to him unfortunately~
(BTW, he will teach every class in UA, not just the main characters, just thought I should let you know that if I wasn't clear, ALSO, Koro-Sensei is given a hero license and is labeled as a quirk user, to not scare the public or raise suspicion of him being responsible for destroying the Moon, plus Koro-Sensei is a very helpful person and will take any chance to help anyone in need)

/Koro-Sensei Abilities\
Skin Coloring: Not only can he manipulate his skin textures and color to express a wide range of emotions, but he can also make himself completely invisible.
(though is shy about being fully naked)

Expert Fighter: His knowledge of the art of combat is so great that he could stun someone just by clapping.
(However, he is more passive and would rather teach others how to fight instead, though that doesn't mean he won't get his tentacles dirty if need be)

Supreme Intellect: He is so smart that he can give AI life, real emotion, and sentience, plus he can make every single individual after images talk on their own without confusion, ALSO, he can even heal and revive the dead with his extreme knowledge of human biology, not to mention he is a top-tier teacher.
(But he does have dumb moments here and there)

Super Senses: Basically advanced sight, smell, touch, hearing, taste, etc etc etc.
(Possible to confuse his senses)

Strong Teeth And Stomach: He can eat and digest almost anything, including non-edible objects like metal.
(Though can't eat everything like fire, lava, and lasers, maybe?)

Unnatural Biology: Besides looking like a yellow emoji octopus, his body is so strange that stabbing his body or shooting him with a gun will only cause the blade and bullet to melt.
(He also has acidic licks, but it's not that strong and pretty much useless to mention)

Regeneration: As long as you avoid hitting his heart or brain, he can regenerate almost all injuries in just a few seconds.
(However it takes energy and stamina, and the more he gets injured, the slower he can regenerate)

Tentacles: A very large amount of tentacles that come in all sizes, shapes, ranges, etc.
(However, if you squeeze one of the yellow limbs hard enough, it will go limp and be completely useless)

Super Strength: Watch the song.
(Due to his squishy body, he can't hurt anyone if he tried to punch them unless he can harden his skin or rely on bitches of slaps)

Protective Layer Of Mucus And Skin: Specialize skin that can protect him from being exploded, burnt, wet, and many more.
(Can only survive one big explosion before molting, he could only do once a month, and can not handle a constant stream or a large amount of water or flames to his body)

Immunity To Most Things: Can not be killed with poison, venom, toxin, disease, and maybe more things.
(We do not know the limit to this his immune system)

Liquid form: Can turn his entire body into a small blog puddle of some kind of gray liquid, while still keeping his speed and movements.
(Can be boiled alive)

Absolute Defense: He has a special defensive form that can even survive a nuke without a scratch.
(Will be the size of a small ball and be unable to move at all)

Antimatter Energy Laser: He can either shoot simple and harmless lasers to push back enemies without killing them into something just as powerful as his Absolute Defense.
(If he goes all out, he will be unable to move for an entire day)

Mach 20 Speed: It speaks for itself, he could probably go faster if he continues to train and learn about himself and with his students.
(If distracted or flustered or beyond confused, his speed will completely drop and make him very unbalance and clumsy)