This takes place after season 6, but your backstory will be before it.

Before any major events, a world known villain would rise up and terrorize countries from all over the globe. This villains name was Biowar, you. Your quirk is biomass. You can control up to the cellular levels of a living being in a 20 feet radius, though, it took incredible mental processing power, so if you strained your brain too much, you’d bleed through your eyes, and if you’d continue, you’d suffer damage that would take you months to recover from; so you’d stay away from controlling a full human being as well as multiple at a time.

To play the villain, you’d act as if you had biological weapons to insert fear into those who tried to stop you. Quite frankly, you were bluffing. The only real toxins you had were sleeping gas, tear gas and some more harmless chemicals.

Many nicknamed you the selfish Robin Hood, since you only stole from the wealthiest of companies and people. What confused people is why go only for the rich? You’d travel the globe and stash your riches on a secret location that only you know.

You settled on Japan with the intent of stealing from a greedy gaming company. For the first time, they cracked your codes. European organizations of hero’s, who cracked your movements, sent your next probable location to the Hero’s of Japan and set up an ambush. You were captured.

Fast forward to after Shigarakis awakening, they had to, reluctantly, let you out under supervision. Your powers can save thousands of lives. Since your powers work in the cellular level, regrowing people’s limbs and other organs tend to be a breeze as long as you get the necessary rest. You agreed. You weren’t really a true villain, just greedy, so you never really injured anyone in your career, and now, redemption? Your first patient is Miruko, who lost so much of her body. You’re briefed on her condition And many doctors were surprised by your knowledge in the medical field, but you had to for your powers to be used effectively.