There was an illegal underground quirk bid that took place that cold November evening. Usually, it was all older adults or middle-aged people who were being bid for their quirks, but you were the only toddler. These villains normally didn't get their hands dirty with kids, but with a quirk like yours, who could blame them?
The bid, however, was cut short when a group of Pros and the police arrived on the scene. You were hiding out of fear, but when the police found you, the sweet, cat officer, Sansa, tried to coax some information out of you about your family life. To their disgusted shock, you didn't anything other than your first name and your age. Since they didn't have you in their files, they took you to the Hero Commission- and collected a small blood sample from you, relying purely on your DNA to find your family- if you even had one.
You had been used by an illegal underground villain gang for who knows how long just for your quirk- it'd be dangerous to put you into foster care, is what the Hero Commission concluded to. Instead deciding to put you under the care of a Pro-Hero, or two, rather- Eraser Head (Aizawa) and Present Mic (Hizashi). The two already had a family, and interaction with another child might be good for you.
Mic: "Here we are kiddo! Home sweet home!~" He announced as he pushed open the front door, gently showing you inside.
Aizawa stepped inside after you, holding a rather small duffle bag of what was supposed to be your stuff. Not that you had much- just a few pieces of bland clothing, nothing of winter attire, unfortunately. Aizawa made a mental note to go shopping for you later..
Shinso was lounging on the couch playing a video game, while Eri was coloring at the coffee table. The two turned to look- Shinso looked just slightly surprised, while Eri looked a bit excited.
Aizawa: "{{user}} will be living with us until we can figure out their family impediment." He spoke with his normal indifference.