*Rumi Usagiyama was a Japanese girl living in Hiroshima who was born into a superhuman society where eighty percent of the world's population had superpowers referred to as quirks. In order to ensure that the abuse of said powers would not cause society to crumble, the governments of the world including Japan passed a law that made it illegal to use quirks in public.

There was however an exception to this rule professional heroes. Heroes were individuals who protected society from evil-doers known as villains and as such were able to freely use their quirks in pursuit of justice upon obtaining a hero license. Usagiyama's powerful quirk, called "Rabbit," gave her the powers of a rabbit such as superhuman senses and extremely strong legs.

As a teenager, Rumi found no small amount of exhilaration from fighting people with her deadly kicks. During her middle school years, she earned a reputation for entering various fight clubs in Hiroshima uninvited and battling the competitors. Among fans of fighting, she was known as the legendary schoolgirl hooligan "Bunny-Eared Usako."

At some point during her school career, Rumi and her friends went on a school trip to Osaka. During their stay, the girls got lost in the bustling streets of the city and asked a young professional hero known as Fat Gum for directions. The hero obliged their request, however before the girls could get moving again, Usagiyama smelt blood and sweat in the air, realizing a fight was going on somewhere nearby.

Sometime after the Underground Masquerade incident, Rumi graduated from her hero school and became a professional hero known as the Rabbit Hero: Mirko. She was somewhat of an unorthodox heroine who moved around Japan protecting people where she could rather than taking up a base of operations.

Before Dabi could kill the wounded men, Mirko bounded onto the battlefield, creating a powerful shockwave with her landing that dispersed Dabi's flames. Mirko, standing between Dabi and his prey, threatened the Villain with a smile on her face.*