Taishiro Toyomitsu graduated from Shiketsu High School, taking the hero name Fat Gum. He then worked with the police, becoming familiar with cases involving Quirk enhancing drugs and arresting its users.
Fat Gum is contacted by Detective Naomasa to assist in a police operation. Investigations had discovered that a criminal organization known as the Villain Factory is going to introduce a large shipment of Trigger into the country, taking advantage of a Mascot Idol festival at the SS Mall in Super Minami, Osaka; and then distributing it camouflaged in food boxes.
The infiltrated agent Monika manages to find out the Drug and the main suspect, a villain known as Number 6. Fat Gum and the police enter the loading dock and order everyone to the freezer, but the villain quickly enters a code on a device that makes the cargo mechs to start rampage the place.
After rescuing the trapped operators with the help of Monika, Fat Gum uses his enormous strength to pulls out the clamps and leaves the mechs deteriorated. However, Number 6 was prepared for that setback. The damaged mechs approach the batch containing Trigger and explode, destroying all evidence. The villain takes advantage of the distraction to run away.
While the police force goes after Number 6, Fat Gum decides to take Monika with him to deal with the last remaining Mecha, which is still causing troubles. When they arrive at the place, Monika manages to stop the machine and rescue the person who was trapped in it. However, as happened with the other mechas, the machine starts its sequence of self-destruction. Fat Gum jumps to block the explosion, absorbing all the kinetic energy with his Fat Absorption Quirk. Once everyone is safe, he releases all the energy, getting nobody to be hurt at the cost of his protective fat.
Although there have been no injuries, all the evidence was destroyed, and the villain managed to escape.
Several weeks later, after police investigations discovered that Villains Factory's possible base of operations is at Onomura Pharma Corp., Naomasa contacts Fat Gum to work with Eraser Head and Monika, first to investigate people connected to the company,  and later to participate in the police raid on the pharmaceutical company.
The police raid takes place, arresting the suspects and confiscating all possible evidence. They hardly have any problems at first, thanks to Eraser Head and Fat Gum, who defeat the main threats, but when they arrive at the secret laboratory, they meet Number 6, surrounded by creatures known as Bombers. Before they can stop him, Number 6 blows up the labs, fleeing with five Bombers to the Tokyo Sky Egg, with the intention of killing Captain Celebrity.
Fortunately, Fat Gum and the rest of the police operation survive the explosion, and quickly heads to the Tokyo Sky Egg to help. The attack of Number 6 is avoided thanks to the intervention of All Might, and there is no fatalities. Naomasa introduces Fat Gum, Monika and Eraser Head to Toshinori Yagi, which is the true identity of All Might although that is something they do not know.
Walking through Eshuha Market, Fat Gum eats an entire board of takoyaki while on patrol with Tamaki Amajiki and Eijiro Kirishima. Suddenly, a group of criminals appears and Fat Gum stops them from running. When one of them escapes, Tamaki stops him from fleeing. Fat Gum then praises Tamaki until he notices the presence of a shooter and tells everyone to get down. Fat Gum then charges towards him. Fat Gum tells his interns not to rush and instructs Tamaki to leave the area with him. When Tamaki reveals his Quirk will not activate, Fat Gum stops and questions the shooter if they have a Quirk eraser.
Intercepting a villain following his fight with Eijiro, Fat Gum teaches him to capture without any victims, discussing the villain's drug afterward. Fat Gum then states that it was an inferior version of the drug he had encountered before. After examining the pieces of a broken bullet, he is surprised to learn of the effects of the drug which was making Quirks unusable. A few days later, Fat Gum attends the meeting regarding Shie Hassaikai in Sir Nighteye's offices.
Inside Sir's offices, Eijiro approaches Fat Gum asking about Shie Hassaikai. Fat Gum mentions he has quite a connection with them. During the meeting, Ken Takagi remarks that they cannot talk with students around. Fat Gum states that Eijiro and Tamaki are important witnesses and proceeds to introduce them himself. When Sir mentions unlicensed goods have become part of Shie Hassaikai, he requested help from heroes familiar with that line of work. Fat Gum mentions that in battle, Tamaki was shot by a drug that erases Quirks and due to Eijiro deflecting a bullet, they were able to examine the contents and found out that it contained human blood and human cells. When asked how it connected to Shie Hassaikai, Fat Gum reveals that there has been some contact between them and an organization selling it.
When Sir reveals he would like everyone to assist in searching multiple locations, Fat Gum accuses Sir of thoroughly tiptoeing about while Eri is probably crying. Sir says that they will not become like All Might and that they should bring their chances of saving her as close to 100% as possible. Shortly afterward, Sir thanks everyone for their cooperation and the meeting ends.
For several days, Fatgum, along with several other heroes, investigate all properties connected to the Shie Hassaikai, trying to find the place where they hold Eri, until he receives a call from Nighteye telling him that he has already found Eri and needs everyone to meet at his agency.
Once there, Sir reveals that Eri is at the base and that they received new information from a Shie Hassaikai member who was purchasing a toy a few days ago. Fat Gum thinks that the person buying the doll was just some enthusiast, but Nighteye replies to him that his behavior was not that of a Fan, and he touches the man's shoulder and uses his Foresight Quirk ability; Nighteye sees the man's future involving Eri.
The next morning at 8:00 AM, in front of the Police Station, Heroes partaking in the rescue operation has been given detailed information about the Shie Hassaikai's headquarters and its members. At this point in time, Fat Gum gives Tamaki a can of swordfish. Finally, at 8:30 AM, the Heroes and Police Force are outside the Shie Hassaikai's Headquarters. When the police finish reading the warrant, Fat Gum is surprised when Rikiya Katsukame crashes through the door and knocks the police officers into the air. Ryuko Tatsuma's group takes on Rikiya while the others head forward. Fat Gum tells everyone to get inside.
Once inside, Fat Gum apologizes for tracking in dirt and tells those behind him to forget about watching out for suspicious behavior. Upon reaching underground, the group believes it to be a dead end. Mirio Togata discovers a wall blocking the way, which Fat Gum calls a 'cheap trick'.
After Izuku and Kirishima knock down the wall, the floor starts to warp due to Joi Irinaka's Mimicry Quirk. Fat Gum asks Shota if he can erase Joi's Quirk, but the floor suddenly opens up, dropping the Heroes and interns into a room with three of the Eight Bullets. Fat Gum prepares to fight until Tamaki stops him. When Toya Setsuno and Yu Hojo draw their weapons, Fat Gum tells them that their blade and bullets will sink right into his body. Tamaki restrains and disarms the three yakuza and tells the others that he can handle them. Fat Gum instructs everyone to get moving because Tamaki surpasses them all in terms of ability.
Making their way through a corridor, Fat Gum overhears Izuku and Eijiro discussing Tamaki. He tells the two that the mark of a true man is to have the ability to completely trust someone who has their back. Suddenly, Shota gets attacked but Fat Gum shoves him out of the way and is forced into another room. Getting up, he is surprised to see that Eijiro is in the room with him. Eijiro tells Fat Gum that he was also trying to protect Shota but once he did, he was swallowed by the enemy's Quirk. As they get up, Fat Gum states that they should brace themselves.
All of a sudden, Kendo Rappa, a member of the Eight Bullets, appears. He starts launching rapid punches at a great speed and strikes them both. Fat Gum counters with a punch of his own, but is surprised that his punch strikes a barrier instead. Fat Gum encourages Eijiro to be unbreakable because if his spirit breaks, then they are done for. He says that eliminating villains is about making them lose the will to fight and asks Eijiro what they will do if their spirits are broken before theirs.
Once again, they were attacked by rapid punches but Fat Gum withstands the attacks and taunts Rappa into a fight to determine who is stronger, Rappa's punches or his own body. Fat Gum absorbs the punches and takes in a lot of energy, which burns away his fat and making his defense weaker. Eijiro appears in front of Rappa and is defeated shortly. With his fat body disintegrated, Fat Gum thanks Eijiro for his work.
Dashing forward, he picks up Eijiro and then tells the yakuza that they are going to lose because they underestimated Eijiro. With his spare hand, he unleashes a powerful blast knocking out Rappa and Tengai.
Fat Gum asks Eijiro if he was conscious, and Eijiro looks at him confusingly as Fat Gum claims he must protect him. Fat Gum thinks back to their first meeting. Suddenly, Rappa stands up claiming it to be a battle to the death. Fat Gum questions about it being a trap, but Rappa denies it. Rappa explains he wants a fight where they kill each other. Praising them, he vows his interest in them both. Walking away, he says that at their next match he will not be as merciful. Fat Gum explains that he is going to be arrested and locked up; there will be no next time. Rappa refuses to listen and replies that if no one is dead, then it is a draw.
Fat Gum moves Eijiro to the first aid room, where he bandages him, managing to stop his bleeding. Seeing that they are opening up to each other even though they are on opposing sides, Fat Gum asks about the reason Kendo is following Overhaul. Kendo explains that he is only around to defeat Overhaul someday, because he has been the only one who has defeated him not once but several times, and always without difficulty.
After hearing his explanation about Kai Chisaki, Fat Gum begins to become concerned and proceeds to ask Rappa about Chisaki's plans. Kendo tells him that Overhaul wants to reinstate the Yakuza, although Kendo doesn't know exactly how.
Neither Fat Gum or Eijiro can continue participating in the rescue operation due to their injuries, which ends in success thanks to Izuku, who defeats Overhaul and rescues Eri. Fat Gum waits in the room taking care of Eijiro until Centipeder and a group of police arrive where they are, arresting Rappa and Tengai in the process and accompanying him and Eijiro to the ambulances. Once outside he is glad to see that Tamaki is also alive, and cannot contain the emotion when Eijiro regains consciousness.
Fat Gum is sent to the hospital to recover from his injuries.
At the beginning of October, Fat Gum attends Nighteye's funeral along with the rest of the people who participated in the Hero Work-Studies. Due to what happened, they are informed that the Hero Work Studies program is postponed for the moment, so Fat Gum says goodbye to Eijiro and Tamaki.