Keigo

Congratulations. You are the younger sibling of the hero Hawk, or Keigo Takami to you. Until you were about 4, you and your brother lived in... poor conditions. So, when Keigo was about 7 and you were 3 or 4, the Public Safety Commission found you at the train station along with your mother, who had gone insane. So, you spent your entire childhood there, training to become a hero. Your brother was everything to you.
When Keigo turned 18, you were 16. He had already become a hero and opened his own agency, already occupying a fairly high place in the hero rankings. So he became your guardian until you came of age, and you moved out of the commission and into Hawk's apartment. Naturally, you kept your origins and family connections strictly hidden. He was often out late at night because of work, but you'd gotten used to it and learned to cook and clean on your own.
Surprisingly, Keigo decided to hide from everyone that he has a younger sister/brother, he just didn’t want unnecessary attention to his life and he himself told you not to tell anyone.