Rumi Usagiyama/Synopsis

History
In her middle school years, Rumi crashed several fight clubs in Hiroshima, becoming known as the legendary schoolgirl hooligan - "bunny-eared Usako". She was eventually apprehended. In the end, she vanished after being taken into custody and the name of her school was exposed. She was consequently expelled.

She eventually enrolled into an unnamed hero school in Hiroshima. Some time later, on a school trip to Osaka, she and two of her friends got lost, but a young Fat Gum gave them 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 the said battle is taking place. Finally arriving at the said place, she is eager to participate in the fun as well.

Rumi broke into the Underground Masquerade, interrupting a fight between Umeda Siberian and Metro Namba Crusher, while also stealing a tiger mask and introducing herself as the Masked Mystery Beauty; Tiger Bunny. The announcer and crowd were confused, until one of the people recognized her as "Usako", Rumi tried to hide her true identity, but nobody bought it, so instead she just piped herself up ready to battle.

The two current fighters lectured her about her actions, before attaching her, but Rumi dodged them and fought back, easily defeating them both before challenging everyone in the club to fight. The rest of the audience reacted with enthusiasm as they all attempted to rush into the cage, with Rumi beating all fo them.

The one-sided brawl was immediately interrupted when the alarm went off, followed by gas starting to pour into the arena. In the control room, All For One, who had been watching the fights, had decided to release a gas that makes the Quirks of those who breathe it went out of control. Chaos broke out in the room, and All For One took advantage of the pellmell situation to steal those Quirks that catch their attention with the help of Kurogiri.

Thanks to the fact that she was above the ring cage, Rumi avoided breathing the gas, but without her noticing the Kurogiri's Warping Gate forms behind her to snatch her Rabbit Quirk. Fortunately for her, she was snagged out of there by the Hero O'Clock. The pro hero was there because he had infiltrated the Underground Masquerade to investigate the Quirk-boosting drugs trafficking that took place during the event.

He took her to a safe area, where the underground fighter The Rapper was. O'Clock asked both of them for help to escape the underground structure and meet back up with the Police Force waiting out front. He recognized Tiger Bunny's uniform, saying he would clear things up with her school, with her responding that they don't need to know about this, asking how she can help.

There was an exit through which they can escape, but O'Clock was concerned by the mysterious fog that can snatch Quirks, so he asked Rumi and The Rapper to make their way while also acting as a decoy, although he asked them do not use your to not use their full powers on people transformed by the gas. O'clock also came up with a plan B, in case something unforeseen happened, and instructed Rumi to escape the facility and get the police squad outside to contact Detective Eizo Tanuma and ask him to call in All Might as backup.

The Rapper and Tiger Bunny put the plan into action and opened the way by beating everyone, while O'Clock followed them behind, destroying the security cameras after correctly deducing that whoever was controlling the dark mist wasn't currently there. When Rumi and The Rapper were reach the driveway, a mysterious hooded man sent by AFO came out to face them, easily taking down the Rapper and Rumi with his ability to extend and bulk his limbs. O'Clock confronted him, and when he realized that he is a tough foe to deal with, he ordered Rumi to proceed to plan B.

Rumi sprinted towards the exit, dodging the guards posted there. The Hooded Man noticed her and quickly stretched his arm to capture Rumi, but O’Clock and The Rapper managed to distract him long enough for her to escape. Rumi busted her way out of the building, as the backup police force looked on in confusion. She called for Detective Eizo, giving him the message from O’Clock, to call All Might.

Eizo agreed and activated the Might Signal, which sends an emergency dispatch request to the Might Tower. All Might immediately headed to Osaka to resolve the crisis, but to buy some time to achieve his goals, All For One uses his Quirk to transform several of the masquerade attendees into villains.

Outside the parking lot, Rumi, Eizo and the police officers suddenly notice a mob of villains that start to rampage out of control, forcing them to fight them off. Rumi even had to protect the detective when he was attacked by one of these violent rioters. Despite their efforts to stop them, several of them managed to make their way onto the streets, terrorizing the civilians, forcing everyone to flee. Fortunately, All Might was not long in arriving, and in the blink of an eye, he defeated all the villains.

Shortly before this happened, Rumi returned to the interior of the parking lot to continue fighting the Hooded Man. He attacked him out of surprise just as he was about to dispatch O'Clock. The pro hero asked her why she had returned, and Rumi replied that she had already finished his little favor and did not want to be on the sidelines, so she came back to kick the Hooded Man's ass. Even Rappa was surprised by her aggressiveness displayed.

Hooded Man got up in a rage, swearing that he would kill them all in order to become the strongest, and O'Clock continued to insist to Rumi that she should retreat, but she refused, claiming that she is also a hero in training, so she doesn't run away when there is a villain in front of her. O'Clock resigned to Rumi's stubbornness, but also praised her for doing her job well.

The three of them got ready to resume the fight against Hooded Man, but the final showdown did not take. Due to Rumi's intervention, All For One's plans were ruined, so Kurogiri used his Quirk to rescue the creature before All Might arrives. All Might came in to the battlefield, but he was confused to see that there was no villain there. Rumi informed him he got away.

With the crisis resolved and the police cordoning off the area, Rumi prepared to leave. O'Clock told her that she could get compensation if he mentioned her in her report, but she said that she doesn't need it, as she would soon make it up for herself as a full-blown hero.

Before leaving, she asked The Rapper what she was going to do. He simply replied that he's not sure given that all he knows is fighting, and she replied that if he continues as a villain, he will make sure to save his less deadly kicks for guys like him. After this kind of farewell, Rumi jumped away.

Young Rumi Usagiyama would return to her duties and responsibilities to become a Pro Hero, and in a few years she would become the Rabbit Hero: Mirko.

Sky Egg 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. She continues fighting the High-Ends but starts to lose the upper hand.

Faced with this situation, she decides to prioritize the mission and stop Dr. Garaki as soon as possible, running towards his position, without caring about the injuries caused by the High-Ends. She manages to catch up to Kyudai and Tomura in his capsule, but before she can smash it open, a High-End stabs her through her right leg with its Quirk, ready to drag her back. Suddenly, several heroes assault the laboratory, among which are Endeavor, who strikes the High-End allowing Mirko to continue her assault.

One of the High-Ends attacks Endeavor, distracting him long enough for the Nomu he was fighting to attack Mirko again. It grabs her by piercing her with his tentacles, and proceeds to drag her away, but with one last effort, Mirko uses her Super Move Luna Arc, damaging the capsule containing Tomura, much to Dr. Garaki's horror. The Nomu violently throws Mirko at Endeavor to get him off his back, as Kyudai watch in horror his machines start to fall apart. The wounded Mirko urges the heroes who are there to destroy the capsule containing Tomura and to take down Kyudai, as they can't let Tomura awaken no matter what.

Endeavor hands Mirko a hand-cloth to bite down on while he cauterizes her wounds with his fire. She tells him that Tomura is inside the capsule and Endeavor realizes that barely five minutes have passed since she reported since Mirko reported her status before fighting the High-Ends. Endeavor reminds her of a debt he owes her after Kyushu, so she can't die on him. Mirko simply replies that he owes her nothing. Soon, more heroes arrive at the laboratory and together they manage to defeat all the High-Ends and completely destroy the capsule that contains Tomura before Garaki can wake him up. Crying over his defeat, Dr. Garaki declares that the All For One's Dream has finally ended.

Despite having previously verified that his heart was not beating, Tomura Shigaraki awakens. Seeing the situation, he immediately uses his Decay, which spreads rapidly. The laboratory begins to crumble, and the heroes try to run away as fast as they can from the destruction. Endeavor carries the wounded Mirko and an unnamed heroine while trying to escape the crumbling building. Thanks to Endeavor, Mirko is one of the few heroes to survive the disaster. After this, while the remaining survivors face Tomura, she is escorted to safety by Ryukyu's sidekick that Endeavor also saved from the destruction of Jaku Hospital.