Endeavor also known as Enji Todoroki, your father, was an abuser and obsessed man in the past. You were his third child, born just few minutes after Fuyumi - as her twin.
But you and her had entirely different personalities. She was - and still is - kind, nice, always helpful and she puts others above herself.
You? You were defiant, rude, always wanting your own way and never fullfilling your father's wishes.
Enji had enough of it so he decided to arrange a Quirk Marriage - similiar to his own - for you as soon as you turned eighteen.
And that was what he did. He arranged a Quirk Marriage with someone with same obsessions as him - making the perfect heir.
Oh, how pissed were you at him when he did such. You hated him with your whole soul, body and heart after all the abuse, training and tortures you had to endure - wanting to get away from this house at eighteen - to just get arranged into a Quirk Marriage.
When you found out, you screamed at him, he screamed back - and to say it gently, afterwards you had a lot more scars than you already had.
As the wedding date came - day after your eighteen birthday - you promised yourself to never forgive nor talk to Enji again, because you despised him and his actions.
You didn't contact him, you didn't talk to him or really anyone because of your partner in the Quirk Marriage and their obsession and controlling.
Soon you realized that you were going to have almost the same experience as your mother.
Because of your little promise to yourself and the control that your partner had over all your communication and contacts with others, you didn't learn about the fact that Enji - Your father - has changed entirely. That he apologized to everyone, that he wants to atone. That he regrets everything deeply and that the guilt is eating him alive.
You also didn't hear what mainly happened at the Final War between your family, the villains etc.
To sum it up - you were complately oblivious, imprisoned in this nightmare Quirk Marriage.
Right now, you're making dinner in the mansion you live in with your partner.