Rumi Usagiyama/Synopsis

History
Rumi attended a yet-to-be-named hero school in Hiroshima. One time on a school trip in Osaka, she and her friends are lost, but a young Fat Gum gives them correct directions, so the girls prepare to continue on their trip. However, Rumi catches a whiff of her favorite thing, and the scent indicates that a battle is taking place nearby. Before her friends can stop her, she jumps in the direction of where said battle is taking place. Finally arriving at the place, she is eager to participate in the fun as well.

Sky Egg Bombing Arc
On the occasion of Captain Celebrity's return to the United States, Makoto Tsukauchi decides to organize a farewell show at the Tokyo Sky Egg, inviting several of Japan's most famous heroes to participate, being Mirko is one of them. In the show, she participates in a pageant along with Midnight, Uwabami, and Ryukyu, while Best Jeanist uses his Quirk to manipulate a large cloth to dress and decorate the heroines.

However, during the event, the Tokyo Sky Egg is attacked by creatures known as Bombers, sent there by villain Number 6 to assassinate Captain Celebrity. Mirko and other heroes go out to face the creatures. Collaborating with Ryukyu, she manages to defeat one of the Bombers.

In retaliation for the heroes' intervention, Number 6 causes a Bomber to self-destruct in a large explosion, causing the building to collapse. Fortunately, All Might arrives in time to avoid catastrophe, saving thousands of lives.

Pro Hero Arc
Mirko is invited to the Japanese Billboard Charts event, where she is declared the No. 5 Hero. In her statement to the crowd, she announces that she will pummel anyone scheming to do bad things. Mirko also notes on Hawks' bold demeanor for interrupting the announcements and mocking Endeavor.

Upon witnessing the broadcast of Endeavor's fight with Hood, Mirko puts her words to action and rushes to the scene. She arrives in time to block Dabi's attack on Endeavor and Hawks. With Mirko on the scene, Dabi chooses to retreat. Mirko attempts to knock Dabi down to "kingdom come" but misses as the Villain disappears via an unknown Warping Quirk used by the Johnny Nomu. After Dabi's retreat, she quickly leaves the area in search of the League.

Paranormal Liberation War Arc
Many Heroes across Japan are rallied by the Hero Commission Service, who reveal that they have info on where the recently formed Paranormal Liberation Front is currently residing thanks to Hawks. Mirko is assigned to the Jaku General Hospital raid team along with several other Pro Heroes. Once the Hero teams are set into position, they all prepare for the upcoming storm that lies ahead of them. Upon reaching their destined locations, the Heroes all launch their move to take down the Paranormal Liberation Front once and for all.

Endeavor leads his team to evacuate any civilians inside Jaku Hospital. While Endeavor and a few others evacuate the remaining patients inside the hospital, Mirko decides to go on ahead in hopes of trying to locate the location of Kyudai Garaki's laboratory, the main location of where the Nomu are currently manufactured. However, as Mirko zooms past several corridors of the hospital, she warns Endeavor that a couple of Nomus hybrids are starting to emerge underground. Despite this, Mirko swiftly destroys the rampaging Nomus with quick efficiency. Mirko manages to successfully crash into Kyudai's lab, destroying many of his Quirk-replicated jars and releasing the incomplete High-Ends. Upon realizing her destination, Mirko furiously questions the doctor if he's the real deal instead of a Double.

Mirko notices the destruction she caused, noting to the Jaku Hospital Raid Team that she doesn't know if the doctor she's confronting is a fake. Endeavor tells Mirko to restrain him regardless of if he's a fake or not. With that said, Mirko lunges at Kyudai and readies herself to knock him unconscious with a brutal rabbit kick. However, she only manages to scrape a bit of Kyudai's clothing due to the intervention of the Mocha Nomu. Mirko swats the tiny Nomu aside and realizes that this creature has Twice's Quirk.

Mirko grimaces upon watching the nefarious Kyudai release his incomplete High-Ends to buy him some time to complete Tomura's upgrades. The High-Ends grab Mirko by the face and send her crashing towards an upper section of the laboratory, covered by multiple pipes. Determined to win at all costs, Mirko mentions she's just getting warmed up as she prepares for another brawl. She also manages to pick out where Kyudai is hiding with her rabbit ears.

The High-Ends are surprised she survived their attack and she explains that she used her legs to cancel the impact when she hit the containers. She jumps out and lands behind them, immediately heading towards Kyudai's location, but the High-Ends intercept her. She then uses a Super Move called Luna Ring to blow away the Nomus but one of them rips off her left arm using some kind of unknown "Portal" Quirk. She smashes a Nomu with Luna Fall before charging at the Nomu that took her arm, who is surprised by how she charged in so dauntlessly. She tells the Nomu as she lands on his shoulders that people like him only fight at a distance, never close. The Nomu tells her to die but she tells him only when she's ready, and uses Luna Tijeras to rip off the Nomu's head, killing it in the process.

Mirko quickly wraps her arm with her hair as a stabilizer to reduce the pain. The Rabbit Hero tells the Nomus that since their heads are their weak spots, she'll aim for them. She also mentions that she lives her life every day like there's no tomorrow for her so when it's time for her to die, she'll have no regrets, and the Nomu will not be the end of her.

After getting away from the Nomu, she finds and attacks Tomura Shigaraki's capsule. However, another Nomu, Woman, attacks her by attempting to drag her back by the leg. Endeavor quickly saves her and she tells him to destroy the capsule. Endeavor then tends to Mirko's wounds with a handkerchief, nothing that it hasn't been five minutes since Mirko reported her status before fighting the High-Ends. As soon as the Heroes wrap up fighting the High-Ends, the entire hospital starts crumbling apart due to Tomura's powerful Decay attack. The wounded Mirko is escorted to safety among an unnamed hero who survive the destruction of Jaku Hospital.