Mahoro Shimano/Synopsis

History
Mahoro is a little girl who lives on Nabu Island. Because her mother died when she was very young and her father often has to work off the island, she and her younger brother, Katsuma, have been cared for by their neighbors. Being many times without her father and being the older of the two siblings, Mahoro is responsible for the well-being of Katsuma, being quite overprotective with him.

My Hero Academia: Heroes: Rising
Due to Hero Work Recommendation Project, students from Class 1-A of U.A. High School were sent to the island of Nabu to do the hero's job. Katsuma is enraptured by the presence of so many heroes on the island, but Mahoro is quite disdainful towards them, and she wants to show her little brother that heroes are no big deal.

For her first mischief, Mahoro calls the Hero Office that Class 1-A has on the island, asking them to help her "find" her brother Katsuma, who was "lost" somewhere on the island. After the "request", she begins timing the heroes to find out how long it takes them to find her "missing" brother. Izuku Midoriya, Ochaco Uraraka, and Kyoka Jiro search the island and find Katsuma at a nearby park after an hour. Mahoro appears and scolds Izuku for taking so long to find them, revealing the prank. Then she leaves the place with her brother.

Later, Mahoro gets Katsuma and herself some ice cream while making rude comments about U.A. High's students and their ability to respond to danger despite the island's low crime rate. Katsuma doesn't seem bothered by his sister because Izuku was kind to him, so Mahoro formulates another plan to make her brother change his mind about the heroes.

For her next mischief, she will simulate a villain attack. Mahoro creates a gigantic praying mantis with her Hologram Quirk at the ancient castle ruins located in Mount Shiroyama, and sends her brother to warn the students of the presence of a "villain". Before long, Katsuki Bakugo appears with Katsuma in his arms. Katsuki confronts the mantis and uses Stun Grenade to blind it, but he notices something wrong and stops attacking. Izuku arrives shortly afterward and sees Katsuki generating a huge explosion that makes Mahoro fall from her hiding place. The giant mantis disappears, confirming Katsuki's suspicions that the monster was fake.

Katsuki is furious as he shouts to the girl if she's trying to make fun of him. Mahoro tries to play dumb but Katsuki doesn't fall for that one, and angrily tells her that he's aiming to be number one and doesn't need stupid kids to play pranks on him. Katsuki scolds her and Katsuma with an increasingly angry face for having faked a villain attack, forcing Izuku to restrain him for fear that he will hurt the kids. Katsuma and Mahoro scurry off while Katsuki struggles to get Izuku off him.

The kids end up in a sugar cane field as they look at the stars. Mahoro complains about Katsuki, saying that even though he wants to be a hero he seems more like a villain, reinforcing her dislike for heroes because they're all talk. Katsuma tells him that despite that those two boys went to help them. Mahoro is aware that her little brother is fond of heroes and wants to be one, but she says their father is better than any hero.

The next day, after apologizing to Izuku for the mischief of the previous night, and after Izuku encouraged him to follow his dreams, Katsuma reunites with Mahoro to tell her about becoming a hero. But at that moment Nine and his gang arrive on Nabu Island and begin to destroy everything. Terrified, Mahoro contacts Class 1-A headquarters to warn about the attack, but Katsuki answered the phone. He doesn't believe her at first but Izuku hears the exchange and takes the phone. Just before Izuku can answer back, the phone service is cut off.

After warning the heroes, Mahoro and Katsuma run towards their house, but when they arrive, it's destroyed by Nine, who had come to the island for them, or more specifically for Katsuma, since he wanted to steal his Cell Activation Quirk. Mahoro tries to protect her brother and uses her Quirk, Hologram, to create a giant Chinese lion stone that attack Nine, but the villain knows is just an illusion and continues to advance towards them.

Before Nine can steal Katsuma's Quirk, Izuku arrives and rescues the kids and gets them to safety. Izuku faces Nine, but the villain is superior to him thanks to his multiple Quirks. Mahoro and Katsuma try to help Izuku, but he orders them to flee. Nine proceeds to shoot a purple laser at Izuku's chest, wounding him. Frightened, Mahoro uses her Quirk to project a gigantic cartoony version of the wounded Izuku to alert everyone on the island about their whereabouts. Thanks to this, before Nine can disposes of Izuku, Katsuki arrives and engages combat with Nine, but even he is no match for Nine.

Izuku and Katsuki collaborate together to defeat Nine, and realizing the danger they pose to him, Nine uses his Weather Manipulation Quirk to strike them with a lightning bolt. With students down, Nine goes after the children, but then, he suffers from his cell degeneration disease and is unable to continue fighting. Slice arrives to help her leader and Nine orders her to take Katsuma, but Koji Koda, Mezo Shoji, Tsuyu Asui, and Ochaco rescue Izuku, Katsuki, and the children.

Night comes, and Class 1-A students regroup and shelters everyone in a factory to treat the wounded and provide humanitarian aid to the local residents. Izuku and Katsuki are both unconscious, and their wounds are so bad that the doctors says they need to be sent to a hospital in the mainland. Fortunately, Mahoro and Katsuma come into the room and they say that Katsuma can help using his Cell Activation Quirk to heal both Izuku and Katsuki. The doctors appreciate his help, and invite him to heal both heroes.

For hours, Katsuma uses his Quirk on Izuku and Katsuki, trying his hardest to heal them, to the point where he almost passes out. Mahoro asks him to get some rest but he refuses to give up and continues using his Cell Activation Quirk, until finally both Izuku and Katsuki wake up restored.

Class 1-A held a meeting to discuss their plan going forward. Knowing Nine is after Katsuma's Cell Activation Quirk, all Class 1-A members vow that they will protect him, his sister Mahoro and all the inhabitants of Nabu Island until reinforcements from the mainland arrive.

Izuku devises a plan of action for the class: by relocating all the civilians to Mount Shiroyama in the smaller island where the ancient castle is located, it can hold all of the residents in the large cave safely. This way, the villains only have one route to get to Katsuma and everyone has an escape route if needed. The heroes will engage the villains using the terrain to their advantage and spit the villains apart. Mashirao Ojiro, Kyoka, and Mezo will personally guard Katsuma and Mahoro.

When the students see the villains approaching their position, they put their plan into practice. Yuga Aoyama and Momo Yaoyorozu start attacking them from afar with their respective Quirks at full power, managing to separate Nine from his allies. Then the students take turns attacking him as they had planned. Despite their attempts to stop him, Nine gradually defeats the students, until it's Izuku and Katsuki's turn to face him. After a tough fight, they finally get Nine to his limit and the backlash of overusing his Quirk kicks in. In his despair, the villain takes a heavy dose of a Quirk empowering drug and goes beyond his limit, using his Weather Manipulation Quirk to summon a massive lightning, striking Izuku and Katsuki.

The situation for Class 1-A is growing dire as Nine was proven to be too powerful for all of them. Mashirao, Mezo, and Kyoka flee with Katsuma and Mahoro to the escape route in the caves. Mezo takes the kids away while Mashirao and Kyoka ambush the villain first to hold him back but they are smashed by his Hydra Quirk. Nine later catches up to Mezo and attacks him using his purple lasers as Mezo protects the kids. Mashirao and Kyoka catch up to attack Nine with Mezo tackling with his Dupli-Arms Quirk, but the villain blast all three of them away, defeating them.

Mahoro tries to protect her brother, but Nine holds her hostage to demand Katsuma to surrender his Quirk or he will kill his sister. Seeing his sister in danger, Katsuma pulls the courage out of him and runs towards Nine to save her. Nine tosses Mahoro to the side and tries to steal Katsuma's Quirk. Before Nine can steal his Quirk, Izuku and Katsuki enter the fray to save Katsuma and Mahoro. As Izuku kicks Nine's face, Katsuki grabs Mahoro before she falls the ground and puts her down. Mahoro is glad to see him alive, and Katsuki replies that he had told her that he will be the number one hero. As the kids flee to safety, Deku and Katsuki proceed to fight Nine.

This time they have the upper hand in the battle. On the verge of defeat, Nine uses the entire empowering drug to amplify his Weather Manipulation Quirk to the maximum and summons a massive storm, defeating both students. The storm is so powerful that it begins to destroy everything on the island. Mezo manages to regain consciousness and uses his body to shield Katsuma and Mahoro from falling debris.

Izuku and Katsuki managed to survive, but they are severely wounded. Nine proceeds to decimate them with his Bullet Laser and uses his Hydra to start crushing them to death. All hope looks like it's lost. The entire class is down and wounded, and neither Izuku nor Katsuki are able to break free. But then they hear Mahoro and Katsuma cry out to them, imploring them to not to give up. Hearing their cries, Izuku makes the desperate decision to transfer the One For All to Katsuki.

With both owning the One For All, Izuku and Katsuki face Nine again, and this time they defeat the villain once and for all. A few hours later, after Class-1-A emerged victorious over the threat, a field force of professional heroes and the military arrived on Nabu Island, rescuing everyone, medically treating Class 1-A, and providing humanitarian aid. Katsuma and Mahoro thank everyone for saving them.

A few days later, Mahoro and his sister are reunited with their father, and Class 1-A continues to stay on Nabu Island to help repair the damage done by Nine and his villains until the Hero Public Safety Commission suspends the program and orders them to be sent home. With the island being repaired by the military and the heroes, Class 1-A boards a ferry to leave. Katsuma and Mahoro run to the port to say their good-byes to Class 1-A for protecting them and saving the island.

Final War Arc
While Mahoro continues to stay at Nabu Island with her father and her brother, she is unaware of the Final War being fought between the last remaining Heroes of Japan and an army of villains led by All For One. She started to watch a live broadcast of her friend, Izuku, fighting against his archnemesis, Tomura Shigaraki to the death from La Brava's livestream and the war from news outlets who are also live-streaming Gentle's broadcast as people around the globe watch the fighting, now knowing that it's not over yet.