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"You see this extra joint here on the x-ray?"
"What do you mean?"
"I'm sorry but you don't have a quirk."
Something about that felt wrong to Izuku. Maybe it was just the sadness he felt but he wanted that statement to change.
When asked years later, he would never know why he said it. He'd answer that he just had an odd urge to. Whatever the reason, this was the first time he'd say the two words that would shape his life.
"New Choice"
"Congratulations, you have a quirk."
This didn't satisfy him, so he said the two words again.
"New Choice"
"You have a dormant quirk that will awaken 10 years from now."
"New Choice"
"All Might will give you his quirk eventually but it will break your bones when you use it."
"New Choice"
"You have a quirk that is so powerful, it's on par with those self-insert fanfictions that has a protagonist with an overpowered quirk and no downsides"
He deemed this answer acceptable, so reality did too. Unfortunately, reality changing to one's whims, is quite shocking.
"..." Silence.
"I would like to know what you did, young man. I didn't intend to say any of that. Your x-ray also seems to have changed," the specialist said calmly
"Izuku... What did you do!?" Inko Midoriya said not-so-calmly.
"I don't know, mom!!!" Izuku Midoriya said definitely-not-calmly
I know you may be a bit confused. Don't worry, I'll explain. Let's rewind.
This is Izuku Midoriya. A hero fan who wants to save people with a smile, like All Might, his hero. Izuku's mother, Inko Midoriya, has a quirk called Attraction. This lets her attract small objects. Her quirk has no impact on the story but I thought I'd note it anyway.
Izuku's father, Hisashi Midoriya, is a comedian. His quirk is Improvise and it has a bigger effect on the plot than Attraction. This made him adapt to situations quicker than most people. With this quirk, he made a great name for himself in the comedy industry. He was a very well known stand-up and improv comedian. Unfortunately, he died before Izuku was born. Why is his quirk more important? Well, Izuku's quirk had to come from somewhere.
Izuku has a friend, Katsuki Bakugo. Their mothers are close friends so they knew each other since before the womb. A week ago, Bakugo awakened his quirk, Explosion. Since then, he kept questioning Izuku, Auntie Inko, and Mitsuki (his mother) when his "sidekick's" quirk would come. Just to get him to calm down, Inko took Izuku to a quirk specialist to ask what Izuku's quirk is.
That leads to the scenario before you, two Midoriya's and a remarkably calm quirk specialist wondering what the heck happened. Now, I'd normally take you back to where we paused our story but truth be told nothing very interesting happened after that day apart from a few tests so let's fast forward to the interesting part.
"So, basically...you can take 10 lines or action per day and make the person who said it improvise a different way to say the line or do the action until the the quirk finds it funny and the thing it chooses will have repercussions?" Bakugo said slowly.
After a week of tests, befuddlement, and confusion, Izuku decided to go toward the Bakugo house to share this news with Katsuki.
"Pretty much, yeah" Izuku agreed.
After a moment, Bakugo began laughing.
"HahahahahahAHAHAHAHA!!! STUPID NERD GOT A WEAK QUIRK!!! HAHAHA!!!"
"It's not a weak quirk! New Choice!"
"HahahahahaHAHAHAHAHA!!! STUPID NERD GOT A STRONGER QUIRK THAN ME!!! HAHAHA!!!"
There was a loooooong moment of silence. Then...
"...WHAT DID YOU DO, DEKU!!!!!"
"AAAAHHH! NEW CHOICE!!!! NEW CHOICE!!!!!
"...WHAT DID I DO, DEKU!!!!!"
Still scared, Izuku went through a few more choices.
"...WHERE'S YOUR MOM, DEKU!!!!!"
"NEW CHOICE!"
"...GO HOME, DEKU!!!!!"
"NEW CHOICE!"
"...STOP IT, DEKU!!!!!"
"NEW CHOICE!"
[Three Minutes Later]
This entire exchange has been going on for the past three minutes and has attracted an audience of Mitsuki and Inko, who she called after witnessing the hilarity ensuing in front of her. They were both laughing at the pomeranian's misfortune.
Then Bakugo surprised everyone by saying a line completely out of left field.
"I'LL JUMP INTO THAT LAKE OVER THERE, DEKU!!!!!"
Note, that I never mentioned a lake before now. That is because there was no lake anywhere near the Bakugo house. This was probably the reason why Katsuki thought would be a good idea. However, there was now a lake behind the house. Miraculously, this lake did not damage anything around it and the entire neighborhood was simply reshaped to fit this inconveniently convenient lake.
The good news is; the quirk deemed the line Bakugo said funny enough. The bad news...
SPLASH!
The quirk made him keep his word.
"New Choice!"
Worse news is that Izuku somehow thought that this was a perfect time to test out how New Choicing actions work. Maybe he couldn't think straight because he was laughing too hard.
Whatever the reason, Bakugo rose from the lake scowling and dripping. His mouth opened, presumably to yell at Deku for making him do that, when suddenly he turned around did a running start and...
"CANNONBALL!"
SPLASH!
"New Choice!"
Bakugo walked out of the lake still scowling before promptly turning around again. This time climbing a ladder and diving board that inexplicably appeared. He then did a dive into the lake. This dive then turned into a backflip. This backflip then failed and he belly flopped instead. To add insult to injury, Mitsuki and Inko then pulled out two score cards, judging the dive as worthy of only the square root of pi and a crudely drawn picture of a duck. Note that they were a part of the audience and therefore were not affected by the choices in any way, they just had these score cards with them for whatever reason.
When Bakugo walked out of the lake again, one could hear him muttering to himself. 'Please don't say New Choice. Please don't say New Choice. Please don't say New Choice.'
Luckily for him, the quirk spared him from having to think up another bit. Realizing this, he cheered for himself and jumped around a bit and then, by no fault of Izuku or his quirk, slipped into the lake. Needless to say, he never underestimated Izuku again.
And that is the story of how The Undo Hero: Improve-ide discovered his quirk.