Disclaimer: I don't own My Hero Academia or Naruto

A/N: Random plot bunny as usual, and there's probably a dozen plotholes but don't mind. The version of Sasuke I used is the adult one. :v

Oneshot

When had it all started?

This quiet darkness that permeated all of his senses, leaving him drowsy and muddled. A single candle of fire flickered in that darkness, appearing just so innocent and lonely. Sasuke shifted until he was by the fire and placed his hands around it as if protecting it from some unknown wind trying to extinguish it.

And then, he fell asleep again.

The next time he woke up, the world would be a lot colder, and that small fire even lonelier than ever.

—

Shoto didn't know when it had started; when the voice in his mind started. Although rather flat in tone, it was a comforting one in his darkest times and encouraged him to just keep going.

Perhaps it was then?

That day he had finally snapped and his hatred of his old man reached the peak. When he swore to himself that he would reach the top using his ice side alone. He had fainted then, but the next time he woke up, the look his father used on him changed.

He didn't understand the look in his eyes, but it seemed to be wary, like watching a ticking time bomb. Over time, that wariness disappeared until he treated him and trained him just as he did before. But every time he pushed too hard, something would happen and the cycle would continue.

"Leave it to me." The voice would always say as he saw black and fell into dreamland. He questioned it at times, but never really pushed at it.

As he grew older, the voice in his mind also grew stronger.

"Use your fire side, you are going to freeze yourself." The only thing he didn't like about the voice was that it kept telling him to use his fire side, like his old man.

He refused and stubbornly kept telling the voice, Sasuke it had once introduced itself, that he would never cave to his father's demands. Sasuke would always sigh and retreat from him until he had something else to say.

"You are not your father, foolish child. Make the power your own and conquer the top. Limiting yourself is only a foolish slight against your potential."

Shoto continued to ignore him as he trained and worked his way to join U.A. and become a hero.

"It is rather cold in here."

—

Enji looked at the pair of red and purple eyes of his youngest son and a feeling of dread welled up in him. Ever since that day, this version of his son would appear and cause him quite a bit of trouble.

At the start of his appearance, he would usually only pop up when he overtrained Shoto and this personality would appear to threaten him. As Shoto grew older, this personality would come out more often when his son was usually asleep, so it was highly likely his youngest son didn't even know of his other half's actions.

But Enji wasn't willing to do anything about this version of his son either, because he was strong, and everything Shoto wasn't. The rest of the family generally stayed away from this version of Shoto as well, because he was cold, so very cold and the feeling of dread would just well up in their hearts as they looked at his eyes.

But he was still a part of Shoto, and still kind so they slowly accepted him into their life.

They called him 'Sasuke'.

Although cold, his fire burned hotter than anyone. He was willing to do and use anything at his disposal for his goals.

Enji wished Shoto would learn from his other half and use any means for his goals. The rebellion of his youngest son was truly an annoyance.

—

Sasuke watched the fire and protected it. Because the person himself refuses to acknowledge it.

He slowly watched it dim over time, wondering what would happen the day it did go out. And wondering if his reason of existence in this child was to keep this small fire alive.

But things would change, things always would. Whether for good or bad, it was up to fate.

And fate was kind this time.

"Todoroki!"

Midoriya Izuki was just what Shoto needed in his life. A change, a willful and stubborn one to light his fire and change his perception of the world.

Like Naruto had done for him, although it took him a lot of destruction, of both himself and the world around him, before finally accepting it.

"He is good for you."

Shoto acknowledged the voice in his mind as he chatted with Midoriya, but asked, "What do you mean?"

Sasuke never replied, but for some reason, he felt warmer and happier than usual.

Sasuke looked down on himself, he felt like… he was fading? Perhaps the goal and end of his existence is near. Of course what the child he mentally resided in needed was a Naruto. Of course.

—

"It's okay for you to be who you want to be."

As realization sunk in, fire poured from Todoroki, Sasuke felt something click in place.

The blazing fire from his left side left watchers in shock.

"Even though you want to win… Damn it… To help your enemy… Which one of us is screwing around now?" The words that came from his mouth felt both ashy yet hopeful, like he was being released from his own chains.

"I want… to be a hero, too!" He smiled widely as his left blazed and felt a nudge in his mind. Sasuke's voice rang clear, the clearest it had ever been in his mind. "To be a hero, you must save yourself first. You have finally taken the first steps."

Next, the voice of his old man echoed the loudest and he felt Sasuke sigh, and forcefully pushed him aside.

"What?"

"Let me deal with this one last time Shoto, it will probably be my last." And suddenly, in front of Midoriya, Shoto eyes shifted. One into purple while the other into red. Then, he turned to look at Endeavor who abruptly stopped in the middle of his speech.

"He has finally started to accept himself, that is correct. It all starts here for him, that too, is correct." The… strange Todoroki started to speak and the cameras zoomed in on him, causing everyone to gasp as they captured the changes to his eyes.

"But this change will be for himself, so I will say what I always say to you, Enji. Leave him be, he is not your puppet to mold." He turned away from the fire hero and turned to Midoriya, giving him a faint smile.

"Thank you, and take care for me will you?" And like how sudden he appeared, he disappeared and Shoto seems to stumble a bit.

He raised a hand to his head, "Sasuke?"

The voice in his head never replied again, but a feeling of warmth lingered. Shoto understood, the voice that had been with him through most of his life was gone now, just as suddenly as it had come. Well, he should at least win this in his honor?

He let his hand fall to his side. "Sorry for the wait, let's continue where he cut in." Smiles stretched across both of their faces while everyone else was left behind in confusion. Fire and ice formed and a destructive battle was left in their wake.

"Midoriya is good for me, Sasuke."

—

"Hey hey, what happened back then with your eyes Todoroki?" Later on, Uraraka along with the rest of his classmates would ask him.

Todoroki tilted his head and thought about it. "That was… my alter-ego? I think?" What did Sasuke count as in the end? He wasn't an imaginary friend since he could take over his body, he didn't really think he was a split personality either…

"Eh? Whoa, you got a split personality dude?" Kirishima said in shock which was repeated by most of his classmates.

"Ah…" Todoroki answered uncertainly. "He's… gone now though. I think… he was there because of my rejection toward my left half… but now…"

"What kind of person was he?"

"Tell us about him!"

"The fight was really cool!"

"Oh… I didn't really know him…"

—

Later that week, he would receive a picture from his sister. His eyes widen as he took in the photo, one of his siblings, and… Sasuke?

P.S. Sometimes he would come out while you were sleeping. I sent a copy to mom since they never properly met.

A small smile played on his lips, it looks like Sasuke did a lot of things behind his back.