3rd POV:
time: hero training with Aizawa the next day
"We are going to the pool today"
Groaning can be heard throughout most parts of the class except for a few students like Tsu.
"You can't complain, brats. The fewest of you know how to fight in and under the water.
That's why principal Nezu arranged more usage for the hero courses at the pool in the next couple months. Meet me there."
"That is so not fair! The only one being really good in the water is Tsu!" Denki whines on the way to the pool. Mina tried to cheer him up.
"But there are a few more of us who could be pretty good in the water?"
"True. Maybe Shoji could swim really fast and Iida could use his engines under water."
"But Tsu would beat us all, Midobro" Kirishima looks at the mentioned girl.
"By the way, Tsu?"
"What is it, ribbit?"
"Is everyone in your family good with water?"
"Mhm, Sami dives and Satsuki does jet skiing and wave boarding. My father works in a sea business and my mother works in the aquarium, ribbit"
Totally true and I not just wrote it down because it fit :D
"And what did you do before UA, Tsuyu?" Toru asks from behind the group.
"I did synchronized swimming in the same club as my older sister, kero."
"Oh, that's so cool- Hold on." Uraraka raises her hands in a defensive matter.
"Aren't Samidare and Satsuki younger than you? You have another sibling?!"
"And didn't tell us!" Mina gasps offensively.
"She doesn't live with us right now. She has her job as a pro synchro in America. It just never came up."
Tsu shrugs and the group arrives at the pool.
Next to Aizawa stands a tall woman in her mid twenties with dark green hair waiting for the students.
(bit like that but please imagine a bit more similarities to Tsu and their family)
Midoriya gasps.
"No way!"
Bakugo walks up behind his bf.
"What is it, nerd?"
Midoriya jumps in excitement.
"That's Mitsuya Mizu! She revoltionized the history of her sport!"
"...Which is?" Bakugo asked a bit annoyed.
"She swims with a team synchronized to music! They can use their quirks to do amazing jumps and figures in and out of the water! It's the one Tsuyu did before UA."
"How did she revolutionize it, Izuku?" Todoroki comes to join his partners.
Midoriya grins at him.
"She has an air quirk."
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"Listen here, kids"
The class groups around the two adults after some warm up.
"This is-"
"Asui Mizu. Nice to make yer's acquaintance!"
The teacher glares at the woman and she backs away.
"My bad, Shota. You go on."
He sighs and tries starting again but is again interrupted by Kaminari gasping.
"Wait- 'Asui'?! TSUYU?!" The class spins around to the girl in the centre.
There she stands, smiling innocently with her toungue sticking out.
"My sister, ribbit"
"Yeah, no shit. Could have figured." Bakugo rolls his eyes.
"But she doesn't have much similarities with you, though?"
"Hey, don't say that, Kirishima! We can't just assume things!" Momo puffs out her cheeks, then bows at Mizu and Tsu.
"I apologize if that offended you!"
Mizu just laughs to which Tsu giggles.
Aizawa hides a smile and Tokoyami slightly smiles as well. (adoring their partners)
"Don't 'cha worry, mate! I was born after an ONS and me mom didn't want to lose her first baby. Me dad did not see a point in an abortion so I could live.
I seem to have part of me sperm donor's quirk in me with me mom's one.
I don't see meself as a 'mistake' and am thankful I got to be born."
(I wanted to make her language more pirate like and saw a movie where they talked like that T-T
hope it fits)
"She is my sister and I am super happy about that. Nee-san looked after all of us and helped a lot. She is family." Mizu lays a hand on Tsu's head and they smile at Momo and Kirishima.
"You done? Then let's get started. Mizu, you tell them why you're here and I go take a nap."
The class sweatdrops at their teacher's antics and look at their babysitter for the lesson.
"Hah, well... That was weird but expected. Anywho, I am here to help ya use yer's quirks in the water.
I swim in a team of unique quirk users wantin' to defeat nature. Our whole community is full of 'em.
Ya can use fire underwater or electricity, strength or waves. Every quirk can work with water if ya find the way."
Iida raises his hand.
"Yes, hand-chopper. Tell me yer name along with yer question!"
Iida lowers his hand and pushes up his glasses.
"It's Iida Tenya, ma'am."
"Oh, so yer Tensei's lil brother. Well, what ya want to know?"
"Ah yes, I would like to know if you could tell us your quirk along with how you use it in your sport."
"Of course!
"Me quirk is of the air type. It works better if the air around me is humid but it's supposed to work worse in water because of the air part."
Midoriya lifts his arm.
"Yes, lil broccoli" Mizu asks him and the class chuckles.
"She is not wrong though" all of the agree to Shoto's comment.
Flushed, Midoriya speaks up.
"So the humid air part is from your mom and the normal air part from your ...sperm donor? Sorry if that sounded weird but you sound a lot like Sho a year ago." he looks down with a nervous smile and the deku-baku-squad laugh.
"Haha, no problem at all, greenie. And yer right!"
Then Mina speaks up with her arm rised.
"Then how come you can use yours under water? Tsu said that you do synchronized swimming like she did before UA? No offense."
"Good question, pinky pie-"
"It- It's Mina Ashido, please don't call me that."
"My bad, Ash. Is it okay like that?"
"Yeah, that's fine."
"Alright. As I was saying, yer right. I should not be supposed to use it in water but this is because I came here.
I'll try to explain this without science as much as I can. Ya wouldn't be able to understand it."
Denki nods relieved so Shinso bumps his shoulder playfully.
"SO: Water is also known as H2O which means it partly contains oxygen which I can control.
I can control air wich is mostly build from oxygen and carbon dioxide also known as CO2 which also contains oxygen.
If water contains oxygen then that means that I can control it.
For the jumps we do I have to help a bit and use that oxygen to boost me up and out of the water. It also helps to speed me up when I swim."
"...my brain overheated..." Sato sighs to which Mina nods along with Kirishima and Sero.
"...but she explained it well?" Denki tilts his head at his friends.
"Denki! Y-you understood what she meant?" Mina asks, stunned.
"Well, yeah? Those chemical constructions are the easy ones."
From behind Bakugo snorts and Midoriya chuckles along with Todoroki.
"Dunce face, you'll fry their brain cells more with that."
"They are in a shock state right now, let them recover." Todoroki adds.
"Hey, it's not my fault that you helped me train and study!"
"But you asked for it." the greenette deadpans.
"I know and I'm taking full responsibility for that!" he turns back to his friends. "Come on, guys! It's not that surprising!"
"Yes, it is!" and while the old-new dumbass squad (remember the pussycats training camp) cries for their lost member the class turns around to see their hometown teacher rolling towards them or -more specifically- their babysitter.
"Nee-san, I think sensei wants something from you." Tsu tells her sister and Mizu turns around.
"Shota! I thought yer were asleep?" She crouches down.
"Couldn't sleep" the caterpillar grumbles. "You're not having any complications with my class, droplet?"
Mizu blushes at her nickname and beams. "No, no. They are great and so polite! They keep on apologizing for the smallest things... I wonder why?"
'Because now we know that either Tsu or Aizawa would rip our heads off if we offend you' the class thought behind them, sweat dropping.
Back to the two lovebirds.
"That's great, Mizu. Good to know they don't make any chaos. However, I wanted to warn you."
"Huh? About what?"
"Zashi and Nem will come here in the next 15 minutes. They wanted, I quote "support their new partner on their first day".
"Awww, me partners are? Well then I should do me given job the best!"
Shota chuckles at the enthusiasm of his fiancé and smiles warmly.
"You'll do great."
Mizu grins excitedly and stands back up.
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"Alright, ye landlubbers! I want ye to think of a way to use yer quirk in the pool. Let's see what ye got on the spot."
A few raised their hands.
"Aye, monkey, what'd you got?"
"Ehm, it's Ojiro. But anyways, I think I can use my tail like a fish tail or the one from a dolphin?"
"Aye! That is a great idea. Go on and try it out."
"Next is multi-arms."
"Mezo. I can use all my limbs and the extensions from them to swim faster."
"Aye, go try it out."
"Next is... are... floating gloves?"
"My name is Hagakure Toru! My quirk is invisibility. It's a mutation quirk that deflects light so I seem to be invisible. What can I do?"
"Hmmm... That's a difficult one. Could ye give me some time? I promise to give ye an answer!" Mizu answers, smiling brightly.
"Alright, Mizu-sensei! I have your word."
"Can I go next?"
"Aye, Earphone Jack"
"...You know my hero name? But it is Jiro Kyoka."
"Aye! I am in contact with ye elders."
"Wait what?!"
"Thee help me with the music for me shows" Mizu shrugs. "Anyway, ye can use ye quirk to boost ye forward. In water ye weight should be down to around six pounds. Ye quirk can move that much weight."
"Can you speed that up, Mizu? You know their quirks already. It will take too long for everyone to come up with an idea."
Mizu turns around a bit confused but smiles as she understands that the students should start working.
"All right! Then we will do it quick. I'll say ye names and how is one way to use ye quirk."
"I'll start at the beginning: (I don't know to every quirk, i'm sorry T-T)
Laser belly - use ye laser to push ye forwards
Ash - create a dissolving acid to not the pollute the water or just water and shoot ye forward with the force
Tsu - ye already know
Tensei junior - boost yerself with ye engines
Uravity - make yeself lighter to be faster
Lightning bolt - transform ye electricity into energy to move in the water
Red head - try to change ye substance to swim on the surface like wood
Anima - call water animals to help ye out
Cake eater - just use ye strength
Tape - try to shoot ye tape through water and get it attached on any surface to pull forward
Fumikage - use Dark as a second force
Candy cane - push ye with both sides and use ye arms to control ye directions
Toru - ye can use ye deflecting to make ye visible and show ye position; it will either distract or help to rescue
Pomeranian - ye already trained with water with the Pussycats; use the force that ye explosions create to boost
Brokkoli - use OFA to power up ye movements
Creati - build anything ye need; be creative as everything could help
Shinso - have ye ever tried to control animals? ye could try to control them like Anima; and try the capture weapon
I think that was it. Now try it out, everyone. I just came up with those ways so ye might have to change 'em."
"Yes ma'am"