Katsuma Shimano/Synopsis

History
Katsuma Shimano is a little boy who lives on Nabu Island. His mother died when he was very young and his father usually works outside the home, so from childhood the neighbors took care of him and his sister Mahoro.

My Hero Academia: Heroes: Rising
Due to Hero Work Recommendation Project, students from Class 1-A of the U.A. were sent to Nabu to do the hero's job. Katsuma is enraptured by the presence of so many heroes on the island, although his sister is quite disdainful towards them, and forces his brother to participate in several of her mischief.

In the first of her antics, Mahoro calls the U.A. Hero Office to ask them for help to find her lost brother. Izuku, Ochaco and Kyoka search the island and find Katsuma at a nearby park after an hour. But soon it is shown that everything is a fake when Mahoro appears and scolds Izuku for taking so long to find them, revealing that she was timing Izuku and the others on their arrival to their location. 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 this occasion, they will simulate the attack of a villain. Mahoro creates a gigantic Mantis with her Hologram Quirk at the ancient castle ruins located in Mount Shiroyama, and sends her brother to warn the students about it. Katsuma arrives at the Hero Office, interrupting Izuku and Katsuki conversation to warns them about the "threat". Katsuki rushes over to where Katsuma said the villain is located, carrying the scared kid with him, and being followed by Izuku.

Katsuki arrives at the place and confront the mantis, but he quickly realizes it is nothing but an illusion created by Mahoro. Katsuki angrily scolds Mahoro and Katsuma for having faked a villain attack before Izuku restrains him while the two kids run away. Both siblings end up in a sugar cane field, where Mahoro complains about Katsuki violent behavior. Katsuma tells him that despite that unpleasant development, Izuku and Katsuki 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.

Next day, Izuku comes across Katsuma again, who apologizes for his sister's prank last night. He says his sister was trying to prove that the heroes wouldn't come help but they did just like Katsuma believed. Deku notes that Katsuma has an Edgeshot pin on his bag, and asks him if he likes heroes. Katsuma says that he really admires Heroes and wants to be a one even though his Quirk isn't suited for combat. However, his sister dislikes heroes and wants to discourage him from ever becoming one. Izuku realizes that Mahoro doesn't quite hate heroes, she wants to stop Katsuma from every thinking of becoming one because she was afraid that hero work is dangerous, which reminds him of how his mother, Inko, was worried about his well-being.

Izuku assures to Katsuma that anyone can be a hero as long as they focus on putting their effort, and asks him what kind of hero he wants to be. Katsuma tells him that he wants to be a strong hero who can faces the villains. Then Izuku tells Katsuma that he wants to be a hero who who win to save, while he wants to be a hero who save by winning. Izuku clarifies that although it is slightly different, the objective of both is the same, and asks Katsuma to try hard to fulfill his dreams.

Izuku's words cheer Katsuma up, and he leaves to join his sister. By sunset, Katsuma meets back with Mahoro at the same park where he met Izuku and is about to tell his sister about becoming a hero when Nine and his companions arrives and attack the island.

After warning the heroes, Mahoro and Katsuma run towards their house, but when they are about to 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 using her Quirk hologram, but Nine is able to see through her illusions.

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. Katsuki also arrives to fight Nine, and even collaborates with Izuku, but both are defeated. With Izuku and Katsuki 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 his leader and Nine orders her to take Katsuma, but Koji, Mezo, Tsuyu, and Ochaco rescue them.

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 the medical staff from the hospital clinic are able to close their wounds, but they cannot treat them any further because they've suffered so much damage that they need to be sent to a hospital in the mainland. Fortunately, Mahoro and Katsuma come into the room and they say that he 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 Deku and Katsuki, trying his hardest to heal them, to the point where he almost passes out. His sister 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 start waking up.

Class 1-A held a meeting to discuss their plan going forward. Momo created a drone to send an SOS out to the mainland but won't arrive for six more hours. They tried to figure out what the villains are after when Katsuma come into the meeting and reveals that Nine is trying to steal his Quirk. Some students suggest helping the boy escape to thwart the villain's plans, but others advise against that plan as the villains could kill the inhabitants of the island in retaliation. Katsuma then offers to surrender himself to Nine to save everyone, but Izuku appears, having fully recovered after Katsuma healed him, and dissuades the boy of his idea by telling him he has an amazing Quirk that he should never give up.

Katsuki also enters the room fully healed, and tells everyone that the only thing to do is to crush the villains. Inspired by Izuku and Katsuki’s words, the class thinks back of all how their hard work they've done for the people and is now determined to do the same as Izuku and Katsuki. The whole class, unanimously agrees to fight the villains and protect everyone on the island. 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, Kyoka, and Mezo will personally guard Katsuma and Mahoro.

The next morning, after recovering from his illness, Nine, Slice, and Chimera arrive at the entrance to the island after Slice scouted them on the ruins of the ancient castle. The villains make their way to mount Shiroyama, determined to take Katsuma no matter the cost.

When the students see the villains approaching their position, they put their plan into practice. Yuga and Momo 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. Shoji takes the kids away while Mashirao and Kyoka ambush the villain first to hold him back but they are smashed by this Hydra Quirk. Nine later catches up to Shoji 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. Izuku kicks Nine's face, and the force of the blow throws the villain several meters away from Katsuma. Then he apologizes to the little boy for the delay and praises him for his bravery.

Izuku and Katsuki fight Nine again and this time they have the upper hand in the battle. On the verge of defeat, Nine uses the entire empowering drug ito amplifies 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. Shoji 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. While healing Koji, Mahoro and Katsuma implore 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. Later, he helps Recovery Girl to heal Izuku and Katsuki from their injuries using his Quirk. And a few days later, Mahoro and his sister are reunited with their father.

Class 1-A continues to stay on Nabu Island to help repair the damage done by Nine and his villains until the 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.

Katsuma promises to get stronger enough to be enrolled in U.A. High School in the future.

Final War Arc
While Katsuma continues to stay at Nabu Island with his father and his sister, he 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. He 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.