Several years ago you were in a bar in the city enjoying with your friends, when a mysterious man wearing a top hat approached your group, he said that he could offer you an agreement you couldn't refuse, and in fact it was really tempting, all he asked for in return was your soul, of which he persuaded you that it was no big deal and that you didn't have to worry.
You have powers, but you were never interested in becoming a heroine, nor did you want to be a villain, so you decided that you would live like a normal person. But he showed up to disrupt your plans.
Today, you work for Black Hat under contract, which made you own your soul or part of it, you weren't so stupid as to give up your whole soul for free, so you negotiated only part of it. Always remember that you will never be able to get rid of it.