Hope it's good; it felt kind ooc. Also seems like the pairing shifted from Hitoshi/Mei to Hitoshi and strong coffee. Also, I headcanon that Shinsou is actually a pretty good artist but he never lets people see his stuff, you know? So that's sorta in here too. Oops. Slight manga spoilers
There were types of people Hitoshi knew he couldn't get along with: morning people, people who took sarcasm seriously, hipsters, and the overly friendly. But Hatsune Mei was in a category of her own in that regard; how they started dating was beyond his understanding.
Soon after he started training with Aizawa, he began to think of his equipment options. Like anyone with a voice quirk, a speaker of some sort was the most obvious option. But hand to hand combat would only get him so far; would it be too weird to copy Aizawa's method of fighting? Oh well, if he had an issue with it, he'd deal with it later. It was Aizawa's choice to mentor him anyway. You reap what you sow or whatever, right? With his teacher's permission, he began to work on some designs.
His inner artist cheered at the opportunity to create something so important and flashy, but he figured out after several discarded drafts, his desire to make something he thought was cool worked against the need to make it functional. He took the most basic sketch with him on his first visit to the support class's workshop. He'd only gotten a glimpse through a crack in the door when he took off, not wanting to be any part of the strange and cruel torture the weird pink haired girl inside was unleashing upon the hero course students. The people he passed in the halls still remembered seeing the "purple general education dude from the sports fest" run past them years later.
The second time he dropped by the workshop, he was saved from the pink nightmare by just a few minutes. The teacher in charge, Power Loader, looked over his ideas, approved them, and directed him to the nearest coffee pot. "It's not fresh, but it's strong. You look like you need it." He drained a mug full of the lukewarm water of life before he left, passing an energetic Hatsune Mei in the hall. He was just close enough to her her reaction to his sketch ("Oh! Such a nice picture of my next baby!") when he decided to drop by again soon. For he coffee of course.
The third time, she was there but too focused on her current test subject to care about Hitoshi. He was able to snag a fresh cup before talking to Power Loader about his equipment prototypes. "The scarf is simple enough; it's a design I know. Unfortunately, Hatsume-san took a shine to your other idea, the speaker one. It'll work great when it's done but...you should be prepared to explode a few times." The pro hero sighed, somewhat resigned to his fate. "The first version should be done by tomorrow at the latest; she's too into the project to put it off for long." He escaped quickly, before she noticed him, but was looking forward to tomorrow. For the coffee, obviously.
The next time was nothing short of a disaster; Hitoshi was glad the weird kid in his class who was still stubbornly trying to become friends with him tagged along. He could sacrifice him if he needed to escape the clutches of the energetic girl. Sure, it wasn't the most heroic move but as Aizawa taught him, survival was the priority if you're fighting solo. Survival cones before nobility or values; throat punches and knees to the groin were better than just letting yourself get beaten down. So what's a bit of old fashion human sacrifice? (Besides, the jerk just wanted Hitoshi to use his quirk for him so he could extort money from hero course kids. Hitoshi figured he had it coming.)
The coffee was even better this time; it was so strong, he thought it might kill him. Still a much better way to go than what that jerk from his class was going through. But even as he contently drank coffee and chatted with Power Loader about other students who transfered to the hero course in the past, Shinsou kept his guard up. He knew that he and Hatsune were incompatible people and he didn't want to get roped into one of her experiments if she eventually got bored of the other kid. "Look, isn't my cute baby fantastic?! It's perfect for a fire quirk user!" She exclaimed seconds before the device caught fire. Perhaps it was too effective for fire-breath Hairou then.
"Is it always like this?" Shinsou asked Power Loader as he put the fire out. Hairou's eyebrows didn't survive but he figured that was the least of his issues. Instead of showing any sort of remorse, Hatsume was already fixing it, humming happily as she worked.
Writing the burnt boy a note for recovery girl (to heal some minor burns), the teacher sighed heavily. "Usually, nobody gets hurt but her. Nor that it even slows her down. Nor do complaints, explosions, and I don't think she's ever had any issues with inspiration!" He complained. "She'll make it far in the hero support world, no doubt, but that personality of hers is a bit..." Hatsume continued to tinker with her 'babies' without a care. "Hatsume! Set one more fire and I'm banning you for a week! I mean it!"
"Sacrifice is needed for the creation of great devices!" She shot back, putting the finishing touches on a contraption Shinsou recognized as being something slightly similar to his original speaker design. Minus the armor, of course. What the hell.
"Why did you add armor to a basic speaker?" He asked a bit accusingly. "I've got a fighting style and capture weapon that works best with speed; that would just slow me down."
"Don't dismiss our baby so quickly!" Our? She grinned widely. "Voice quirk types usually have weaker bodies since they fight better at a distance! Plus, it has several built in speakers! Your voice will reach all over the battle field! Look!" She pressed a switch and to nobody's suprise, sent herself flying into the wall.
"Hatsume! What did I just tell you?!" Poor Power Loader, Shinsou though sympathetically as the teacher fussed over the dent in the wall, even more irritated by Mei's laughter, completely unfazed by the explosion. Without a word, he finished his coffee and left.
The fifth and sixth were the same; Hatsume was probably going to drive that poor man into an early retirement or a complete breakdown. Mei and Hitoshi disagreed on the direction she was going in the speaker project, and he helped himself to their coffee. It had become their after classes routine and they even made a fresh pot when he came by. It was pretty nice.
The seventh visit, Hitoshi hardly noticed when their group of three became a pair of two; Power Loader was grinning and whistling as he left the two alone, the teens absorbed in their conversation. "I've tested it but I don't think I can control a large group at one time; I'd probably pass out."
Like Hitoshi, she didn't bother to add anything to it before draining her mug. "But your quirk is a one trick deal if you focus on just one target. She suprisingly remembered him from the sports fest, where his quirk was exposed. He had to agree on that; his opponent knowing how his quirk worked was a big disadvantage. "They'll stop replying at some point. So I think a multi-target attack would work better in a fight! And to help carry your voice is this cute baby!" She dove into her pile of creations, looking for something in particular. When she didn't surface for good three of four minutes, he washed his cup and left. It was a good day; maybe he could deal with her type after all.
The eighth proved him wrong. He didn't know why Midoriya and his pals were there or why the door was on the floor, but it looked painful so he decided to just get some coffee and go. He nodded to Power Loader as he filled a mug and quickly gulped it down, desperate for his afternoon cafine rush. Nobody else noticed him, not even Mei. The coffee tasted a bit more bitter than usual today.
His speakers got more and more complex the more they bounced ideas off if each other. He couldn't move well with something bulky and the scarf around his neck? A mask would work better then. Normal speakers were incompatible with his quirk? Voice changers killed two birds with one stone! They added that part together on the eleventh visit. She even let him make parts of it by himself, giving it a cooler look. Power Loader stuck around less and less, leaving the pair to their projects.
With his equipment finished on the twelfth visit, they started talking about other things instead, personal things. She was the odd one of her friend group growing up; while her friends wanted to be heroes who were showered in praise and glory, Mei wanted to dig in junk yards, roll in grease, and make the equipment than made the heroes great. Heck, she never really even cared about the heroes themselves, just the equipment.
Sometimes she drew her design ideas on napkins and tissues when she had a good idea but no paper. That's why a lot of them tended to blow up.
Shinsou told her how he grew up admiring heroes but always being cast as the villan in school plays and games. He started drawing because he wanted to at least make a world on paper where someone with his quirk could be a hero like All Might.
He'd recently started to get into water color painting and digital art and would do favors for the people in his neighborhood to fund his hobby. Currently, he was saving up for a good tablet. It was an odd thing to admit but he felt like they were close enough friends by now that he could tell her that sort of thing.
Though it seemed like they were getting along, there were moments where Hitoshi had to fight the urge to make a run for it simply because she was way too hard to predict. He had no idea what she was going to do next and he had a feeling she didn't either. When they became a couple, no more than six visits later, the "how" part of the equation was still a mystery to him. But he supposed it wasn't such a bad thing to be dating her; she was pretty fun when she wasn't blowing things up or setting them on fire.
She was in a category of her own for sure. Hatsume Mei was the number one type of girl Shinsou Hitoshi had no idea how to deal with, even when they were getting along. So of course he married her.
In his defense it was damn good coffee.