Mt. Lady made her first debut as a new superhero just as Kamui Woods was about to restrain his opponent with his Lacquered Chain Prison Super Move, when she swooped in, attacking the giant villain with her Canyon Cannon move, and subsequently defeating him. She then introduces herself to the awestruck crowd, proclaiming it to be her first day on the job, and is afterward swarmed by photographers and paparazzi.
Later the same day, when Katsuki Bakugo is taken hostage by the Sludge Villain, she is unable to do anything because she has no room to move in the narrow alley in her giant form. Mt. Lady uses her gigantic arms to protect the bystanders of the wind pressure caused when All Might defeated the villain with a Detroit Smash. She helps to recover the remains of the sludge villain to take him into the custody of the Police Force.
Three days after her debut, in her Hero Office, Mt. Lady has a discussion with her assistant about the insane bills they have to pay due to the destruction she caused with her gigantic form, even if they are covered by insurance. During the discussion, she accidentally activates her Quirk, demolishing her own headquarters. Afterwards, she says it's time for a rebuild.
She along with Kamui Woods were having trouble dealing with Habit Headgear, as he was holding hostages. That was until All Might showed up and had beaten him and saved the hostages. Although she and Kamui Woods were grateful, they complained that he'll put them out of a job.
Mt. Lady is seen outside the U.A. Sports Festival, having been assigned to guard security. She comments, while eating takoyaki, that all the professional heroes came to watch the event and notes that this year "is gonna be crazy".
After the rules of the Cavalry Battle are explained, Mt. Lady, along with Kamui Woods and Death Arms discuss competition and cooperation in the Hero business. The other hero says that if a hero fails, they lose any chances of surviving, and notes that the Obstacle Race is a perfect example of how sometimes one has to lower someone else to show themselves off, and Mt. Lady mentions that she felt bad for taking out Kamui Woods's enemy on her first day of work, while also asking the other hero to stop smoking, while Kamui Woods agrees with the hero. Mt. Lady then comments that sometimes heroes have to cooperate with their concurrence and that the Cavalry Battle is a perfect analogy to that.
Later, while doing her surveillance duties, Mt. Lady watches the fight between Denki Kaminari against Ibara Shiozaki, in which Ibara easily wins. Impressed by her, Mt. Lady tells Kamui Woods that Ibara would make the perfect sidekick for him, but he only responds by telling her to get back to work.
For the Hero Agency Internships, Minoru states that he wants his workplace to be with Mt. Lady, causing Tsuyu to think that he is having unclean thoughts, to which Minoru denies. Once there, she makes Mineta her servant, cleaning up and doing other menial labor around her office while she lazed around.
Days later, after the internship ended, Denki asks Minoru about how his experience was with Mt. Lady, Minoru answers while quivering that he knows the true nature of women now, indicating that his experience with Mt. Lady was traumatic.
Mt. Lady is featured on a daytime talk show where she appears Midnight who is also being interviewed. While Midnight talks about her costumes controversies, Mt. Lady repeatedly interrupts her by poking fun at her need for attention. Eventually, the two come to blows over their ages, much to Minoru's viewing pleasure.
Mt. Lady joins the Police Force and many Pro Heroes in the rescue operation of Katsuki Bakugo.
At the League of Villains' warehouse, Mt. Lady destroys the front of the warehouse. Mt. Lady along with Tiger, Best Jeanist, and Gang Orca infiltrate the partially destroyed warehouse and apprehend all the Nomus while Tora rescues Ragdoll. Best Jeanist informs Naomasa Tsukauchi that the warehouse has been cleared.
When All For One appears from the shadows, Mt. Lady sees that Best Jeanist is preparing to attack and warns him that the man could be a civilian. Before the Pro Heroes can act, the entire warehouse is destroyed.
Mt. Lady survives the attack thanks to Best Jeanist's quick thinking but is heavily injured. After Izuku and his friends rescue Katsuki, Mr. Compress is flung into the air to catch them. Suddenly, Mt. Lady uses Titan Cliff and intercepts Mr. Compress, which causes them to smash into each other and prevents Mr. Compress from getting to the students. Izuku and Katsuki are surprised at Mt. Lady's intervention. The dazed heroine tells the students to escape before falling down and being left unconscious. Kamui Woods later rescues the unconscious Mt. Lady, telling her she did well.
When the Hero Billboard Chart JP announcement is made, Mt. Lady is ranked No.23. In an interview during the announcement, Mt. Lady comments that being on a team with the No.7 Pro Hero Kamui Woods gives her a lot of encouragement (although privately she is frustrated at not making the top 10 while Kamui did).
When the reporter asks about the rumors of Mt. Lady and Kamui having a romantic relationship, she avoids answering the question. Mt. Lady then cried and ran to Death Arms for comfort about Kamui receiving recognition.
When Class 1-A is learning about how to act in front of media, Mt. Lady is helping since she knows how to act in front of it and tells them to use their special moves in front of the reporters. She tells the class that when they use their special moves in front of the media, they are using it to show villains that it is a warning of their presence, to heroes a key to team building, and to the citizens that they are safe. Mt. Lady then praises class 1-A for doing a great job but when Izuku Midoriya tried he becomes stiff but when she mentions that his power is similar to All Might, he bursts out talking.
Later, The Hero Public Safety Commission demand all the students heroes for all academies to undergo practical field training. Denki Kaminari, Hanta Sero, and Minoru Mineta from Class 1-A, and Ibara Shiozaki from Class 1-B join The Lurkers.
By the end of March, thanks to the police investigations, and the information leaked by Hawks, who was acting as a spy, the Hero Public Safety Commission organize the Heroes and the Police Force to take down the Paranormal Liberation Front. Mt. Lady, along with her teammates Kamui Woods and Edgeshot, is part of a large force of heroes in charge of raiding the Gunga Mountain Villa, the headquarters of the Paranormal Liberation Front.
When the signal is given, the heroes begin the assault on the Gunga Mountain Villa, catching the PLF members off guard, allowing the heroes to defeat large numbers of them before they can mount a proper defense. The Paranormal Liberation Front soldiers soon begin to reorganize to drive back the heroes. In her giant form, Mt. Lady is hit by several of the enemy's attacks, but all they manage to do is anger her and strikes back in retaliation.
As the battle progresses, the PLF begins to offer increased resistance. Geten, one of the main lieutenants of the organization, intervenes in the fight and with his ice he blows away Mt. Lady, which almost falls on top of Fat Gum. Mt. Lady gets back up and attacks Geten, intending to pay him back for his earlier attack.
Suddenly, Gigantomachia emerges from the ground and picks up some of the lieutenants. Mt. Lady does her best to try and hold the giant back. The giant however, ignores her presence, until he chooses the shortest route to Tomura, grabbing her by her leg and throwing her away. Kamui Woods and Midnight also try to stop the behemoth, but are defeated by the lieutenants. However, Midnight manages to convey to the students the key to stop Gigantomachia, and Mt. Lady along with Kamui Woods holding on to his left leg, slowing him down just a little bit, giving students time to set up a trap.
Mt. Lady continues to hold onto Gigantomachia while yelling at the unconscious Kamui Woods to wake up. Then, Gigantomachia falls into the trap, and the students prepare to carry out the plan to take him down, which is to throw a container with a powerful sedative into his mouth. Despite their situation, both Gigantomachia and the lieutenants manage to repel the assaults of the students.
Then several heroes who were chasing them start attacking, at the same time Mt. Lady pushes Gigantomachia down on the ground. Understanding that students trying to get something into Gigantomachia mouth, Mt. Lady forcibly opens it. Mina is about to succeed, but she fails when she see Gigantomachia, who proclaims that he cannot waste any more time with insects. The behemoth grabs Mt Lady by the head and shakes her off, then savagely tosses her to the side, knocking her unconscious. When it seems that the plan of the U.A students is about to fail, Eijiro manages to make him swallow one of the sedative.