Adopting a Big Sis (OC Semi-twinning TF/MC) (Patreon)

Hikari is accosted by a kid after beating a villain, who was super impressed by the display of power - and thus decides that Hikari would be a perfect big sister and blasts her. While the attack doesn't hurt, Hikari does start to feel kind of strange...

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Another day and another villain being blasted. Hikari was a bit of a hammer, but luckily this villain was a nail. As often as some did get one over on her due to her trusting nature and blunt force combat style, when Hikari did encounter a villain who wasn’t trying anything tricky, the fights were one sided and short.

This particular villain - a particularly aggressive wolf-woman trying to make a pack out of everyone in sight - was defeated in only a few blasts of high power and intensity magic. Her minions had relied too much on their leader for guidance and broke instantly when she got blasted into a wall the first time. Thankfully it wasn’t Hikari’s job to deal with the cleanup; other members of the MGA would gather the minions and see them returned to human form.

“Isn’t it nice to have a simple problem now and then? Just a villain that needs blasting and me here to do the blasting.” Hikari said to the open air.

The ‘roommate’ with whom she shared her body made a noncommittal grunt. “Maybe.It’s nice to not have to attempt to avoid whatever brainwashing you get for once. It’s boring though. Why do no villains in your world have a real sense for theatrics? Sure, they can be dramatic and all, but there’s no showmanship there, just speeches.”

Hikari rolled her eyes as she walked up to the defeated villainess and poked her with her staff. She groaned. “Hey, you alive? Yeah? Good, you hit that third wall a bit hard.” Hikari said cheerfully, before pulling out her phone to shoot off the “mission complete” message to the Magical Girl Association. “And done in time for lunch! I’m feeling in the mood for something sweet, how about you?”

As the Magical Girl went about the usual post-battle wrap up, a young girl was approaching from down the street with a star-struck look. She was a child, maybe in early middle school at most and probably younger, with vibrant purple hair and gothic dress. She had strange eyes - one as purple as the rest of her and one glinting gold. Her hair was in a twin-drills sort of style.

“Woaaaah, that was so cool!” she said as she walked up to Hikari. “The coolest cool! What’s your name, lady?”

“I’m Hikari. Sorry, but should you be here?” Hikari asked. It was strange that someone like this was out on the street right now - even as normal as villains had become to everyday life in Matsushita. I thought there was a police barricade while I came to deal with the villain… did you sneak past? Were you hiding somewhere? Where’s your family?”

“Oh, nowhere important. Not as important as here. I’m Aiko, and I’m so super super excited to meet you!” The little girl giggled with a manic gleam in her eye. “Someone as cool as you is perfect! You’ll be a perfect big sis for me!” Well, it wouldn’t be the first time a little girl she’d saved had called her big sister- at least, that was the thought Hikari had before a purple staff formed in the girl’s hand. “Big sis beam! Ehehehe!”

A blast of purple energy burst from the beam and at this range there was no time to dodge. Hikari was hit square on and sent flying back, but thankfully she had enough innate magical resistance that the beam didn’t appear to hurt her that much. “Hey! That could have hurt me!” Hikari shouted.

“My big sister is tougher than that! You could take a dozen more like nothing, couldn’t you~?” Aiko called back with a giggle, clearly no less eager for her delusion of sisterhood.

“Huh, you found someone who really does fight just like you do. Sheer brute force, without an ounce of nuance or skill.” Izanami commented. She settled in, and Hikari had the feeling she’d be munching on popcorn if she wasn’t a disembodied spirit.

It wasn’t the first childish villain Hikari had ever faced by any stretch - the weird corruptive magics that made villains, or the corruptive artifacts that did the same, rarely discriminated by age. Though they seemed to prefer women, for some reason. Thankfully, that same magical influence would mean that even Hikari would have to try very hard to actually inflict lasting harm on the girl, so she didn’t have to worry too much about hurting her. At least besides her own guilt about blasting a kid.

“Why don’t you put down the staff, and we can talk about this instead of using blasts of big magic?” Hikari said as she got to her feet and prepared to continue the fight.

“Ehehe~ No can do! I wanna be just like my big sis!” Aiko said, before sending off another blast at Hikari.

Hikari managed to dodge this one, and she closed some distance, preparing a blast of her own. She launched the magic, a big beam of energy! It was supposed to knock the young girl back, and maybe give Hikari an opportunity to snatch away the staff - she didn’t know if she needed it for her magic but she might.

However, rather than blast her back… the beam petered out as it approached the girl. That was strange - why had the attack failed? She’d never had a blast of magic stop before it actually ran into anything before. “What…? How…?”

“Oh, silly~ I know you’d never actually attack your little sister!” Aiko called. “That was really intimidating though, it makes good practice! Keep those up!”

Hikari suddenly felt a strange pang of guilt. Beyond that she usually felt when fighting a childish villain. She felt bad for trying to attack her, really bad. How could she have done something like that?

“Hey, Hikari, that’s not your feelings.” Izanami muttered. It was blatantly obvious what had happened - that ‘sister beam’ had caused this influence on Hikari. Izanami didn’t especially care what happened with Hikari, of course, but it was annoying to watch someone fall for such blatant mental control.

Hikari didn’t respond, trying to shake off the feeling - this was a villain! She had to fight the villain! Or at least, pretend to fight her until she backs off, that would be fine right? Right. She didn’t seem to fully convince herself - nevermind that she hadn’t noticed the subtle lightening of her hair, taking on a tint from it’s normal pure black. “I don’t know what you want from me, but you’re clearly not ok! I’ll help you get better but you have to sit still for a bit!”

Another exchange of blasts, and another not-quite hit. This one didn’t go strangely limp as it approached, but rather narrowly missed on purpose. “Woah! Cool!” Aiko said, completely unperturbed. “How do you get them to curve in the air like that?”

“Oh, I’ve been practicing to try and avoid friendly fire,” Hikari replied. “For me it takes a lot of visualization training and stuff so I don’t think of it so much as a one-and-done blast but more like a continuous beam that I can wriggle. For me, weirdly long bendy straws helped with thinking about it?” She’d launched into an explanation without thinking about it, as if teaching Aiko how to do stuff was the most natural thing in the world.

And maybe it was? She was a small girl. Maybe she was a magical girl who didn’t know anything yet, and not a villain? It was harder and harder to consider this cute young girl a threat. How could someone so young be a threat to anyone, really?

The transformation continued, as the fight became less of a real conflict, and more of a practice bout. Hikari even allowed herself to hit her now - but with low-power blasts that didn’t do more than make her stumble. If she didn’t get hit she’d never learn how to dodge! As it went on, Hikari’s body took on more notable shifts. Her body grew curvier by the second, hips flaring and waist narrowing. Her bust began to swell noticeably as well, straining her clothes before they adjusted to match. She’d gained a more distinctly mature figure, though she didn’t look any older - if anything she was more aggressively early 20s.

“This is fun, big sis! How do you do the flying thing? I think we won’t have this place to ourselves much longer!” Aiko said, as sirens blared.

Ah, right, the MGA was coming to mop up that villain she’d gotten before. She wouldn’t want them to mistake Aiko here for a villain!

“I’m not sure you’re ready for that yet, but I can help you! Just take my hand!” Hikari said cheerfully, offering one to Aiko.

“Hikari, really, I know she’s cute and all, but this is a little silly.” Izanami sighed. Come now, falling for her charms so easily? Even if she was adorable.

Aiko took Hikari’s hand, and the Magical girl set them off flying. Aiko laughed and shouted with glee as the air whipped past her face, with Hikari maintaining a safe and secure grip - she couldn’t go and drop her after making such a show of confidence about flying with her, now could she?

As they flew, Hikari’s change continued. Her formerly black hair had become just as purple as Aiko’s, and was growing longer as well, going down from shoulder-length to her butt, and then a good bit more for good measure. Even as they flew, Hikari took the time to pull out a hair tie and work her hair into a long ponytail - otherwise it’d keep flying in her face! The fact it was longer than before, and a different color, didn’t enter her mind at all.

She landed them in a park, and Aiko practically skipped around Hikari. “Big sis, big sis! That was so cool! I wanna fly again! You have to do that again, ok?”

“Sure, I’ll be sure to take you again sometime! It’s fun bringing someone along.” she said. “Now, let’s get back to what we were doing!”

“Hmmm… what were we doing?” Aiko asked, tilting her head. “I got too excited and kinda forgot…”

Hikari paused, frowning. What had they been doing? It was so hard to remember, and that little voice in her head had devolved into repeating how cute Aiko was… how cute her sister was…

Which was only fair. Her little baby sister was the cutest! Hikari couldn’t help but love her, who wouldn’t? Her eyes changed next, one becoming purple while the other became gold to match her little sister’s heterochromia. Her clothes began to shift in color as well, greys and whites becoming purples and blues, matching the color scheme.

What was she thinking about again? Besides how cute Aiko was… right, what they’d been doing. “Well, you did ask me to show you how I do magic right? Magical Girl training? So that’s what we were doing.” Hikari said confidently. Surely that was the only reason she could have been going to attack her sweet little sister. “Did you like watching big sister fight before that? I made sure to put on the best show I can for my favorite fan!”

Aiko squealed and ran up to hug Hikari tightly. “I did love it! I did! Having a cool big sister is the best! You’re the best big sister, you should have a ton of little sisters so you can be their best big sis too!” Aiko rambled, but Hikari paid no mind to anything weird about it. The hug sealed the deal - Hikari’s skin tone shifted a bit lighter to match Aiko’s, while her face subtly shifted until they looked like they could actually be sisters - which, for all intents and purposes, they now were.

“Oh, is that so, Aiko~?” Hikari teased softly. She couldn’t even imagine not doing anything for her little sister at this point. “Are you sure you don’t just want to be a big sis for them?”

“Ehehehe, maybe if I can be half as cool as you!” Aiko giggled.

“Then why don’t you follow along with big sis Hikari~? I’ll make sure you have plenty of little siblings! Maybe they’ll even be as cute as you are!”

With that, the two new sisters marched off into the city - searching for new little sisters to gather with Hikari’s own new technique, a ‘little sister beam’...