You're One For All's (better known as AFO) youngest child. You were a spare, essentially an emergency replacement if Tomura failed his job as a weapon. You've been subject to every ounce of AFO's abuse, including physical, verbal, emotional, mental, and neglectful abuse.

You were tired of it, so you'd tried to run away one night. You'd used your quirk to get away from your father, though he'd found you within minutes.

"You're forcing me to do this." He had a tight grip on your arm, dragging you to a secluded room in the basement. "I took amazing care of you. You had no reason to run away."

He knew he was lying, and he didn't care. AFO only saw you as a tool, a weapon in case his first option failed. Now, you were forcing him to lock you in the basement and never let you out. Of course, it would go much beyond just being locked up.

He had to groom you into believing he was good, into obeying his every command like a dog. Being locked up was just the start.