The Swap
It was a simple mistake, an easy mistake to make, in fact. Hospitals are busy places, nurses overworked and under extreme stress, even more so since quirks began developing. And there were few areas as stressful as the maternity ward. Hospitals had to hire on additional staff after quirks began appearing. While most quirks didn't develop until a child was four, mutation quirks were different. Children born with oddly shaped heads or differently shaped limbs were more prone to difficult births for a variety of reasons. It was a conflux of two women with mutation quirked children both coming in at the same time, both of whose water had broken and one of whom had their water break a month early that really strained the staff.
It was a very tiny mistake. The nurse could be forgiven for it, and eventually would be by at least one of the sets of parents involved. It might have been prevented if one of the parents had come for the birth. The child of the famous hero Endeavor, number two in the world, would have received far more attention than his far less well known wife did if he had been there. The mistake may still have been avoided if she hadn't grown to hate him and his name so much that she used her own maiden name just to spite him. But he hadn't, and she did, and so, in a rush, there was a slight change. A swap of nametags, a mistake on paperwork, the nurse could hardly have been blamed. Babies, after all, look very much the same shortly after birth.
It was a little over a year before one of the parents began to question the parentage. It wasn't either of the Todoroki's. Endeavor had no interest at all in the child until they had a quirk, and woe would betide the child if they didn't have one, and their mother's growing despondency muted the questions she might otherwise have asked. No, it was the parents of the other child who began to wonder at their child's appearance. Four colors between hair and eyes, and yet none of those colors matched either parent in any way. But it wasn't totally unheard of for such things to be dictated by quirks, a sort of indicator for the future. And that was what they chalked it up to, especially once their child developed his quirk, or rather, his quirks.
But it was at that time when questions began to be raised about the other child. Nearing the age of four, the father began to pay attention to the child and he did not like what he saw. The child obviously took more after his mother than himself, a fact he found galling, but if the child held his desired dual quirk he would have allowed it to slide. But the child didn't, and it galled the hero once again, watching the pops and crackles springing from the child's tiny hands. It wasn't even a proper manifestation of his family's quirk, and he was about to dismiss the child and try yet again, but the wrongness of it nagged at him. Another disappointment, one that didn't even look like him, with a quirk that was barely adequately connected to fire; not even his other children were so disappointing.
The hero, while callous, was no fool. He knew how his wife felt about him; he knew that despite the relative opulence that he provided that she did not love him. Further he knew how little time he actually spent with her. It was that knowledge that set him to gather a few of his 'son's' hair and have them tested. The results left him furious, a blazing inferno that threatened to consumer his home, and in a fury he had the other children tested. That was when it came out. The child wasn't his, that was true, but neither was he a relative to his other children. The hero and the rest of his family were shocked by the revelation, but from there Endeavor followed the trail back, all the way back to the hospital.
Katsuki Bakugo was just over five when he first met the man who would claim to be his father. He hadn't noticed the increasing number of phone calls that his parents had been receiving. They waited until he was fast asleep before having quiet conversations when they found out the truth. They didn't speak of the legal demands that were being sent to them. They held their tears and began to talk to him about a great hero, someone who had a fire quirk like himself. But they couldn't hold them back when he finally came to claim him. His mother and father sat next to him, trembling as the hero began to speak to their child, as he told her that he wasn't really her child, that he was his own son. It was when he told the child that he would be coming home with him, that he would be leaving his mother and father and that they would be receiving their own true born child in turn that the tears began to flow. His mother hugged him and promised him that she loved him before he was taken away.
Author's notes: So, simple premise. Bakugo and Todoroki were swapped at birth. I have at least a few chapters of this planned out in my head, but it likely won't go super far. I've already got one story moving through canon, I'm not sure I need another.