When Izuku first changes, he doesn't notice until gym class.
He makes to put on his shorts, but when he looks down, he notices that he's missing something.
In most situations, he might have screamed from the shock - or maybe laughed in relief. He has a quirk! But this is gym class, and the teacher is scary. If she heard that Izuku had interrupted class for something as silly as a quirk, she would not be happy.
So he hurriedly finishes getting dressed, feeling oddly uncomfortable in the presence of his oblivious classmates, and waits for Agasa-sensei to lead them to the field.
Running that day is better than usual. He still runs out of stamina far too quickly, but at least the quick onset of exhaustion is no longer accompanied by that uncomfortable friction between his legs.
Once Izuku's mother comes to pick him up from school, he thinks of telling her about the quirk.
But wasn't that part of him supposed to be private? She'd told him that herself, back before school started. It would disappoint her if he showed her that part of him, wouldn't it?
So instead, he acts like his excitement is for watching All Might's awesome rescue mission again - which it is, in part. Once he gets home, his mother sets up the computer and leaves the room. With a silent apology to his all-time hero, he pauses the video, and sets about figuring out his quirk.
"Private parts," he murmurs as he types that phrase into the search bar.
Izuku scrolls through the search results, but not enough of the words make sense to him. He doesn't know which one to click, so he scrolls to the top and clicks Images.
"That one matches," he says to himself, and he clicks on one of the pictures.
"It's odd," the doctor tells them. "He's long past the age at which a quirk would manifest, and yet his toe doesn't have the extra joint to identify him as quirkless..."
"But what does that mean for him?" his mother asks. "He's wanted to be a hero for as long as he can remember!"
The doctor shrugs, eyes falling pityingly on Izuku. "I'm sorry to say that probably isn't possible. Izuku has what's known as an invisible quirk - one so obscure, difficult to activate or with so little usability, that it's impossible to identify. Ninety-nine times of a hundred, having an invisible quirk is as good as being quirkless in a combat situation."
His mother's eyes well up immediately. Izuku's don't.
He knows what his quirk is. And yes, it's small, and yes, it won't be any good on its own.
But Izuku will make it work.
Both the doctor and his mother catch a glimpse of his eyes, the green fire of determination, and they drop the subject then and there.
Izuku can't decide a lot of things, and it's starting to upset him.
Firstly, should he tell his mother about his quirk?
His first instinct is to tell her, but he knows, realistically, that his quirk isn't very powerful. He can't spit flame, make explosions or defeat enemies with a single blow. If he tells mom about the quirk, then she'll know for certain that he can't be a hero.
At least if his quirk is invisible, she won't know for certain that it's useless. Izuku will still be able to dream about being a hero.
So he isn't going to tell mom. It hurts to keep this a secret, but it would hurt more to give up his dream. And he can't decide whether it would hurt more or less than that, to see his mom hurt like she had during the doctor's appointment.
Still, that's just one more thing he can't decide. There's still plenty more to get through.
His eyes drift to the computer screen, the innocent question of 'am i a boy or a girl' typed into the search engine. He thought at first that he was a little silly to be unsure about such a simple fact - something that everyone knows their entire lives - but according to the internet results, it looks like he isn't as alone as he thought.
Intrigued, he clicks on a random link and prepares to sieve through this new information.
Izuku can't really think of themself as a 'they' full-time. It's fine to talk about Izuku with a 'they' when they're talking about Izuku overall, but for most situations, Izuku decides that they'll use 'he' or 'she' depending on which private parts they have at the moment.
It's simpler that way, she nods, with the conviction that only a five-year-old can have.
Smiling at her own logic, Izuku closes the tabs she has open, making sure to remember them for later. This whole 'transgender' thing is very interesting; to think it was her own useless quirk that helped her discover the whole subject!
She's spent the last week as a girl. Not because it's more comfortable to sit and walk (although it is), but partly to see if there are any limits to her quirk - like a time limit. Lots of cool heroes have talked about knowing your limits, after all. But so far she hasn't noticed any limits beyond those of a regular human body, and the strengths of her quirk aren't exactly stellar either.
The other part is that Izuku's curious about whether they're transgender. He isn't uncomfortable as a boy, but she isn't uncomfortable as a girl, either. Izuku thinks they must be 'nonbinary', that extra category that's neither a boy or a girl, except that Izuku is okay being both, so that doesn't really fit at all.
She'd thought that being a girl full-time for a bit would help her figure it out, but it hasn't helped in the slightest.
She's about to snap from the frustration of it all.
Stretching out, she decides that staying as a girl won't get her anywhere right now. So she changes.
And drops to the floor.
His head swam. He threw up in his mouth and barely avoided left a puddle on the floor. His hands feel clammy and cold, and when he rubbed them together to try and warm up, bits of dried skin flaked away. His mouth stuck together, and he can taste copper in his mouth.
Then he was in the kitchen, crawling towards the table and trying, without success, to prop himself up against it. A glass of water appears in front of him, and he desperately butted against it. It went down his throat, and despite inhaling some, he felt a little more alive once he downed the entire glass.
Now he can see his mom, desperately searching through the cupboards for something. Maybe some more water? Water was supposed to come from a tap, how could she forget that.
He told her as much through the water coating his vision, and she says something that doesn't reach his ears. She finally pulls out a packet of something. He gripped onto the biscuit he's given, and stared at it blankly.
How was he supposed to drink this?
The moment he's himself enough to think it through, Izuku changes back into a girl. Her pounding headache lets up a little, and while she's still exhausted, she now has the brainpower to piece together why everything went wrong.
They only really had problems when they tried to stay in their girl form for too long. And the problem itself was starvation and dehydration, which only affected their boy form. So logically, that meant that when they stayed a girl, the boy form wasn't getting any food or drink.
By being a girl for a week, she was letting the boy starve. And by staying a boy to let that side of him recover, he made the girl go hungry a bit, too. That's a big weakness, especially since it probably means that they need to eat enough for two people.
But maybe it could also be a strength, she thinks, hope rekindling in her chest for the first time in weeks. Surely if food doesn't transfer from the boy to the girl, that means that injuries won't either!
Crossing her fingers, she searches for a sharp object. There's a pack of thumbtacks on the other side of the room, so she picks one up and jams it into her left thumb. It stings as expected, but she pulls it out, and there's little left but a sparkling red bead on the tip of her thumb.
She changes, and he squints through the returning migraine at his thumb.
The blood is gone, and none wells up to take its place.
He changes back into a girl. The droplet returns.
She smiles to herself.
As the years go by, Izuku learns about their quirk, little by little. Whenever they have a bit of free time, they try to learn more about themselves.
If they concentrate, they can extend the change to their clothing, so the girl can wear different clothes to the boy. The first time they try this, she ends up naked in the computer room (in girl form, no less), and is almost caught that way when her mother walks in.
Since then, every morning, they undress, shower and dress both of their forms. Every night, they brush both sets of teeth. They don't need to swap clothes whenever they change, but having a set of clothing on each form makes them feel more comfortable. This way, they won't end up naked if they change clothes on reflex.
Their next experiment is to try and store their notepad; if they can store clothing in that way, why not tools? And sure enough, the notepad disappears the moment she changes into a boy. It's almost like a videogame inventory, the way they can just pick something up, store it, and stop worrying about it like that.
There is a limit, they find out when they try to store a huge box full of all their All Might toys. If they can't carry it, they can't store it either. So they can't just pop a car out of existence, and then pop it back wherever they want.
Unless they could pick up a car.
Izuku hums to himself, formulating yet another new idea. Not all of their ideas come to fruition, but he's optimistic about this one.
Their mother is shocked at just how much Izuku's eating, but they've always been one of the smaller kids in their year, so she lets it slide. When she realises that Izuku's actually losing weight despite the mountains he inhales daily, she attributes his newfound appetite to his otherwise-unseen quirk, and it only takes a few weeks to get extra income support so she can feed them properly.
Despite this hiccup in their daily lives, Izuku's mom is happier than ever. Her son's quirk isn't invisible anymore!
When she says as much, Izuku almost can't bear not telling her the truth. But Kacchan's bullying has gotten worse lately, and Izuku just knows that if they say anything to anyone, it'll eventually spread to Kacchan. Having an unknown quirk means Kacchan hasn't written Izuku off completely, but having a 'useless' one? If Kacchan learns about that, he'll be worse than ever.
So Izuku doesn't tell her.
She's changing. Her hips are getting wider. He's getting taller, too, but his changes are much less obvious than hers.
When Kacchan pokes her one day and his finger squishes into her chest, she punches him in the face and runs away, beetroot. Kacchan doesn't follow, and he seems to second-guess that she was even Izuku. The bloody nose was probably very convincing - no way could useless Deku have done that.
His mother notices something different about her when she gets home, but she can't put her finger on it.
Izuku needs to be more careful.
She knows what to do. From now on, the girl is a secret identity.
Izuku's recent haircuts have taken longer than usual, since they'd keep changing back and forth so that both forms got the same haircut.
This time, he stays in his boy form the whole way through. His hairdresser is surprised at how much easier it is than usual, and when they're done, he grins at his reflection.
A few days later, seven sports bras are mistakenly delivered to the Midoriya household. No matter how anyone looks at it, the box is addressed to this home, so the bras end up stored in a cupboard and forgotten.
Izuku takes them. Luckily enough, they fit her girl form perfectly. He has to sneak them into the washing inside other bits of clothing, but his mom never manages to find them, until Izuku gets the bright idea to offer to do the washing himself.
Over the next year, a few more deliveries are made, which happen to coincide with the old ones growing too tight.
Izuku feels slightly less awkward but much more guilty with each theft, but she tells herself that she needs them and sucks it up.
Izuku's mom hasn't seen them as a girl since the haircut. To eat as a girl, he takes longer to eat than usual, and then takes it upstairs when she isn't paying much attention. If he can't escape the table, he changes whenever his mom looks away, and snaps back to being a boy when she turns to face him again. There are many, many close calls and dinner always leaves him ready to drop like a rock, but she never finds out his secret.
As Izuku gets older, his mother starts to notice various feminine products go missing, with coins appearing in their place. She just shrugs and buys more.
Izuku stays as a boy for school and home, but he only spends as much time there as he needs to.
With the extra, they train. He does mainly strength and endurance training, disguised as a cleanup operation for the trash-covered beach he found. She does mostly running and agility. When half of them is exhausted, they just switch to the other, and they can train both their bodies as much as anyone else could train theirs. It takes twice as long to train than it does for most people, but they can withstand twice the injury, so they think it evens out.
Besides, a little hard work never killed anybody.
Every day for months - and then years - he comes home worn down and exhausted - often more mentally than physically - but always beaming. His mother doesn't know what to make of it, but when asked, he just mentions that he needs to be strong to be a hero.
As a boy, Izuku bulks up within just a few months, still fairly mobile but specialising more in raw strength; at least, as much strength as a young teenager can have. As a girl, she stands an entire head smaller than him, and while her muscles aren't nearly as pronounced, she's far faster and lighter on her feet than he could hope to be.
Training continues, and while Izuku's grades slip very slightly, the Hero Course of UA has never seemed closer.
"Oh yeah, Midoriya was planning to go to UA too, wasn't he?"
Izuku, having previously tried to phase his head through his desk and just hide, groans to himself and braces as Kacchan slowly turns towards him.
Then the class bursts into laughter.
"I've never even seen him use his quirk!" one girl in the back exclaims.
"Studying won't get you superpowers, you know," a boy reasons, snickering.
Another one cackles, "you can't get on the course without a quirk!"
"They removed that rule," Izuku argues, figuring that if he wasn't going to go unnoticed, he could at least try to defend himself. "Besides, I do have one, it just isn't obvious right now."
A hand shoots out from Kacchan's direction in retaliation, but he slaps it to the side on reflex, and the resulting explosion makes Kacchan slip from the desk he was crouched on. The raging blond drops to the floor like a sack of bricks, and Izuku's stomach does a little loop at the trouble he's just gotten himself in.
The teacher pays little attention to the exchange, besides smirking at the problem child's misfortune. "Midoriya, perhaps you should think of a different profession to follow, which will better suit your... abilities," he suggests with a pitying look.
Unoccupied by Kacchan, the teacher is unnerved the fire in Izuku's eyes, but Izuku doesn't care. This is not a dream they'll give up on that easily.
Foreseeing an attack by Kacchan, Izuku changes when everybody's scrambling to leave. Kacchan spots her and makes a start in her direction, but then he pauses, growls and heads out the door, muttering about a 'slippery bastard'.
Looks like he bought the disguise. It's lucky he thinks his class is beneath him, otherwise he might have noticed that the nondescript girl sweating bullets wasn't a regular class member.
Izuku relaxes into a grin as she watches Kacchan's retreating back, then glances down at the notebook she'd stored in her girl form.
It's a fairly new one. No. 6 is written on the front, along with a crude sketch of Ingenium.
Izuku's been studying the quirks of several heroes, but after discovering so many subtleties of their own quirk, they've decided to go as in-depth as they possibly can on just their six favorite heroes. Well, they hadn't ever thought of a specific number to make, but every time they ran out of things to write about all the other heroes on their list, they added another notebook.
Naturally, All Might occupies notebook No. 1.
She opens the latest notebook, and her analysis continues from where it left off. Currently she's trying to figure out a way around Ingenium's difficulty in stopping and turning properly - he'd already had two accidents this month, and while he's effective at close-combat due to his quirk-enhanced punches, the ability to use his full running speed in alleyways and other tight spaces would make him even better at chasing down villains.
A bicycle would be too clumsy to use in combat, but it's the first thing that comes to mind, so she jots it down, along with a little argument between her optimistic side and her devil's advocate.
Roller blades or quad skates... a good direction to take, but the wheels would likely wear away too quickly at Ingenium's high speeds, and they'd get in the way of hand-to-hand combat. The material used for his current boots is shear-resistant, but only in certain directions, so wheels made of it would wear away unevenly. Instead, Ingenium needs wheels which only come out when they're needed, like landing gear.
Idea drawn in her mind, she hums to herself as her absent footsteps take her under a bridge, sketching a potential design for the mechanism involved.