Reborn as a different character's twin

Bakugou

I always did my absolute best to keep Katsuki grounded. It was a bit harder after our quirks appeared, but manageable. My quirk is called Time Bomb, in case you were wondering. It was like an inversion of Katsuki's quirk. Where he could secrete nitroglycerin and create explosions with his palms, anything I touched with my palms could gain explosive properties to be detonated at my leisure. Our parents' quirks mixed well together, and no doubt any child they had- though no more would come after the terror of having Katsuki- would have had some kind of explosion quirk.

Katsuki always was a prideful child, but at least his relationship with Izuku was much better than it would've been, all thanks to me, of course. Where in my memories, those two were follower and leader turned bitter rivals, now, with me included, we were pretty much the three musketeers. With the revelation that Izu-chan was quirkless, Katsuki's protective instincts shot up tenfold. Instead of being someone to disparage, he became someone to protect. It was probably because I made sure to pound it into my impressionable toddler twin's head that only villains would hurt someone who was weaker than them. Nipping those abusive, bullying tendencies had been the first thing on my list when I'd realized who exactly I'd been reincarnated as.

That's not to say that I changed him completely. In fact, he was still the same confident, bordering arrogant, aggressive, victory-or-death, hothead from my memories. The more things change, the more they stay the same, I guess. The only real difference was his disinclination to physically assault Izuku, and yes, that's exactly what I told him it was- I wasn't going to sugarcoat or kiss his ass like everyone else; I would not stand for any abusive behavior from him.

Katsuki had some misgivings about supporting Izu-chan's dream of being a hero, but I managed to talk him around. I even got them both started on physical training, something I never understood why the original Midoriya didn't do if he was serious about still being a hero despite his quirkless status. The three of us signed up for martial arts classes, and with Izuku's analytical skills, taking up the styles we felt would best be suited to our quirks in the future. I personally wanted to learn a style similar to this one character I remembered who used her flexibility and speed for precision strikes that would block her opponent's chi and incapacitate them. Izuku agreed it would work well with my quirk since I need physical contact to make something explode.

I was worried that the changes I made meant that Izuku would miss his chance to impress All Might and not gain the power of One for All, but even if he didn't I'd still support his dream. Not to mention, if by the time Katuski and I get into U.A. (Katsuki was a given, and where he went, I followed) and Izuku did not have One for All, I would come clean to All Might and push Izuku his way. He might not have been my twin, but he was just as much my boy as Katsuki was. I'd protect them both.

Uraraka

My twin and I were identical in every way, physically at least. Even our parents could hardly tell us apart if it wasn't for our wildly different styles and quirks. My sister was always the cutesy, trendy one, what with her short hair and sweet nature, whereas I was the "cool" and "mature" twin, with my long, dyed hair and darker clothes. Our parents always joked about our quirks being a reflection of our personalities. Ochako's Zero Gravity representing her lighter, bubbly attitude, and then there's my own Gravity Field, which I can use to make objects within my range as heavy as I want.

I'd known that my parents were struggling with money, so I used my "genius" intellect to help in any way I could. Ochako and I already had plans to be successful pro heroes to support them financially (I had no interest in being a hero, but I wasn't going to let my sweet baby sister get hurt if I could help it), but that would have to wait until we graduated from U.A. In the meantime, I took up an old skill of mine, piano, and passed myself off as a child prodigy, winning various competitions just for the prize money. The sponsorships alone covered enough that I could get Ochako and I enrolled into, ironically enough, Gunhead's Martial Arts Dojo.

Sometimes, I wondered if my parents realized what I was doing. They'd look at me so sadly when I mentioned my music money being able to pay for something that we couldn't have otherwise. I suspected they felt guilty that they had to rely on their eight-year-old for supplemental income. I swore I would take to my grave the fact that I only played piano for the money and not because I actually liked it.

Shinsou

Finding out you've been reincarnated as the twin of a minor side-character is both reassuring and terrifying. Reassuring in that it guarantees your lack of involvement with most of the plot and thus the danger, terrifying in that all your relevant knowledge is essentially useless and you can't change anything as you aren't in a position to do so. Not that I actually want to be in the thick of things, goodness no. In fact, I'd be perfectly happy if Hitoshi decided not to be a hero, but alas, we can't always get what we want.

My stubborn, loveable twin had his heart set on becoming a hero, and as much as I disliked the idea, I wasn't about to hurt him by letting him think I didn't believe in him. With our mind control quirks, we were both regarded as untrustworthy and villainous. He needed someone in his corner who would support him, and support him I would, by helping him achieve his dream. With some subtle manipulation, I convinced him to follow in the footsteps of his once and future mentor, Eraserhead. Like us, Aizawa had a non-combative quirk but still managed to become a successful pro hero, so my twin was practically worshipping Eraserhead as soon as I'd introduced him to the underground hero.

The original Shinsou had relied too heavily on his Brainwashing quirk, that without it, he was easily taken out through physical means. I wasn't about to let either of us fall into that trap and pushed him into training his body. Neither of us would ever be popular heroes. We'd have to stay underground because of our easily countered quirks, so information about them would stay limited. My quirk, Hypnotic Voice, doesn't require a question to be answered like Hitoshi's does, but it would be equally useless if a villain managed to block the sound of my voice. That was fine with me. I didn't particularly care to be in the spotlight anyway, and neither did my twin for that matter, being the introverts that we were.