Gran Torino never wanted to be a Pro Hero. However, as it was the only way to legally work towards a certain goal, he went ahead and got his license.
At some point in the past, Gran Torino was part of the U.A. Staff, All Might's homeroom teacher, and a close friend of his predecessor Nana Shimura. With her, he mentored Toshinori Yagi, delivering brutal "practical training" in which he beat the young Toshinori mercilessly. During one training session with All Might, Gran Torino punched him so hard in his stomach that he vomited. Gran Torino admits having been rough on All Might, accidentally instilling an innate fear within him.
Along with Nana Shimura and All Might, they tried to stop All For One, but he proved to be too powerful. Nana sacrificed her life so that Toshinori Yagi could escape, entrusting his safety to Gran Torino. After that, Gran Torino continued training Toshinori. As he still did not see Toshinori ready to face All For One, he recommended that, once he graduated from the U.A., he should travel to the United States, partly to gain experience as a hero, and partly to distance him from All For One and his agents, who were all over Japan. Toshinori took his advice.
After several years, All Might returned to Japan and in a short time became the number 1 hero of the country. His heroic acts led him once again to face All For One and he defeated him, but suffered serious injuries. Sorahiko was present when All Might dissolved his partnership with Sir Nighteye for having different opinions. All Might refused to retire from hero's life despite injuries, despite the fact that if he continued on that path he would die in 6 or 7 years, as Sir Nighteye warned him.
Following the League of Villains' assault on the U.S.J., Gran Torino's old student: Toshinori Yagi, pens him a letter. In his letter, Toshinori explains that he has found a successor and has passed One For All to a young man named Izuku Midoriya.
Prior to the U.A. Sports Festival, Gran Torino sent out an internship offer to Toshinori's new disciple. When Izuku arrives at Gran Torino's apartment, he finds the old man laying in a pool of sausages and ketchup. He mistakes it for blood and assumes he's walked into a murder scene, but Gran Torino reveals himself to be alive and scares the young man. He continues to mess with Izuku, putting on a fake senile persona and asks for the boy's name repeatedly. He briefly shows his true nature to challenge his new protege to try and strike him with a smash to gauge his adaptation to One For All. Izuku does not take Gran Torino seriously and tries to leave because he believes the old man isn't in any shape to mentor.
Gran Torino drops his façade and stops Izuku by activating his Quirk and using it to jump around the room. He scolds Izuku for using his Quirk recklessly during the Sports Festival, and does the same to All Might, who Gran Torino calls a blunder when it comes to teaching. He tells Izuku that he can't stand to see how Izuku uses his Quirk and tells him if he wants to learn more about One For All, then he should stay and put his Hero Costume on.
Seeing his true persona, Izuku makes the connection between All Might's split personality and Gran Torino's, and decides to take the challenge seriously.
Their duel begins, but Izuku is worried about hurting Gran Torino or damaging his home. Losing patience, Gran Torino uses his Quirk to bounce around the room and hits Izuku in his back several times. He is impressed when Izuku analyzes his movements and tries to counter, but Gran Torino is able to easily evade and pins Izuku to the ground, defeating him.
Gran Torino tells Izuku that his movements are too rigid because there is discord in his mind and his loyalty to All Might has become a shackle. Then he leaves Izuku to ponder his advice while he goes to get food. When he returns from the store, he overhears Izuku realizing that he has to make One For All an extension of himself if he wishes to master it. Impressed, Gran Torino muses over Toshinori's choice for One For All's ninth holder. That night, Gran Torino goes to sleep right after returning with food.
The next morning, Gran Torino finds Izuku slightly injured from training on his own the previous night. He encourages Izuku to keep training and tells him about All Might during his youth. In contrast to Izuku, Toshinori was able to use One For All right away and only required combat training. The training was difficult, but Gran Torino needed to do it because Toshinori was entrusted to him by his late friend. He's surprised to learn that Izuku knows virtually nothing about the seventh holder of One For All.
Izuku receives Gran Torino's new microwave in the mail and they prepare some frozen taiyaki pastries. However, Izuku prepares them incorrectly and angers the old man. Gran Torino berates Izuku, telling him to put the pastries on a smaller plate so they can rotate in the microwave and cook evenly. Izuku eventually realizes Gran Torino is actually making a hint, and the parallel between himself and the taiyaki. He spreads One For All's power evenly throughout his body to fully integrate its power. Gran Torino is impressed with how little time it took Izuku to realize this and decides to challenge him once again to hit him with a smash.
After stretching out, Gran Torino sets the time limit to three minutes for their rematch and kicks the energy out of Izuku right from the start. When Izuku tries to get the power flowing again, Gran Torino doesn't give him an opening to. The old man uses his Quirk to fly around the room and strike Izuku from different angles. Izuku eventually makes his way underneath a couch and sends it flying at Gran Torino to buy time.
Izuku's distraction keeps Gran Torino off him long enough to activate his new technique: One For All: Full Cowl and he leaps at Gran Torino and nearly strikes him. After evading, Gran Torino moves to strike Izuku from behind, but the young man jumps above his sensei and goes to strike him with a smash. Once again Gran Torino narrowly dodges and counterattacks, sending Izuku flying into a nearby wall.
Time runs out, and although Izuku fails, Gran Torino commends him for his effort. He surmises that Izuku spends a lot of his time strategizing and tells the young man he's improved since their previous bout the day before. Unbeknownst to Izuku, he managed to graze Gran Torino's cheek, prompting the old man to think over his student's monstrous potential.
Afterwards, Gran Torino suggests they eat breakfast, but all the taiyaki have spilled onto the floor. This infuriates him, enough so to scare Izuku into running to the store to buy more. Sometime later, they continue training until Gran Torino decides to move on to the next phase, to avoid Izuku developing strange habits from their sparring matches, he wants to take Izuku to fight real villains in Shibuya Tokyo.
They take the bullet train to Shinjuku station and Gran Torino comments that they're going late to have more fun with more skirmishes. They pass through Hosu City when suddenly a strange monster breaks into the train with a pro-hero in its grip. Gran Torino leaps into action and tackles the beast out of the train, through the air and into a building within the city limits.
The Nomu breaks out of the building and attacks a few civilians indiscriminately. Gran Torino does his best to stop it, but the beast nearly attacks two nearby bystanders when Endeavor arrives and stops it.
Despite its burns, the Nomu responds by activating a few of its Quirks. Before it can attack, Gran Torino propels himself through the air and smashes into it with enough force to break the road. With the creature defeated, Gran Torino calls for himself and Endeavor to restrain and turn it over to police before moving to support other heroes in their efforts against the remaining monsters. Endeavor claims that his sidekicks will handle turning the Nomu over to the police while he supports the pros alone, and instead asks Gran Torino to head to a certain address.
Once there, Gran Torino finds Izuku and his classmates prior to defeating Hero Killer Stain. He's angry with Izuku for leaving the bullet train and kicks him in the face, berating him for not staying put like he asked. In spite of his anger, Gran Torino is relieved to find Izuku alive and ponders over the similar personalities of Izuku and his predecessor. Pro Heroes sent by Endeavor arrive at the scene, followed closely by a flying Nomu. Gran Torino warns everyone to get down, but the beast swoops down and grabs Izuku. Gran Torino worries that it will elevate too high for him to pursue, but Stain recovers in time to save the boy by killing the beast.
Endeavor catches up and desires to fight with Stain, but Gran Torino pleads for him to stand down. The villain turns to face Endeavor and his allies, instilling fear into each one of them with his ferocious willpower and bloodlust. The malefic aura only dissipates when Stain falls unconscious standing up, relieving Gran Torino and the others.
The next day at the Hosu General Hospital, Gran Torino and Manual visit Izuku, Shoto and Tenya. He wants to yell at Izuku, but introduces the boys to the Chief of Hosu Police instead. Chief Kenji Tsuragamae informs the boys that their mentors, Gran Torino included, and themselves must receive strict punishment for using their Quirks on Stain without permission. Shoto becomes angry with the police official, but Gran Torino asks him to hear Chief Tsuragamae out. The Chief lets the boys off the hook, but Gran Torino's pay is docked and his teaching license is revoked for the next six months.
Later on, Gran Torino calls Toshinori to complain about Izuku and discuss the Hero Killer. He explains that although he only met Stain briefly, the villain managed to instill great fear deep into his soul. He goes on to say that Stain's fanatic ideology and charisma will likely rally villains behind the League of Villains. Gran Torino surmises that if the leader of the League of Villains predicted this would happen, then he is a brilliant strategist. All Might and Gran Torino agree that it is likely All For One has returned. To conclude their conversation, Gran Torino urges his former student to tell Izuku the whole truth about One For All.
The internship concludes at the end of the week and Gran Torino bids farewell to his young student outside his apartment. Izuku thanks Gran Torino for teaching him to use One For All better, but Gran Torino reminds him to maintain control and stay within the five percent limitation.
Izuku asks Gran Torino why he never became a famous Pro despite his strength. The latter admits to never wanting to work as a Pro Hero and claims he only got his license to work towards a certain goal. He tells Izuku to ask All Might to fill him in and then returns to his senile state. He asks Izuku his name, and finally, Izuku realizes the answer his mentor is looking for. Izuku replies that his name is Deku, garnering a nod of approval from Gran Torino. The old man returns to his home and muses about the day when the name Deku will be celebrated as the new symbol of peace.
Two days after the Vanguard Action Squad abducts Katsuki Bakugo, Gran Torino is recruited to join the Hideout Raid Team. This team is tasked with infiltrating the League of Villains hideout in Kamino Ward. Prior to the mission, Gran Torino asks All Might why he was recruited alongside so many other Pro Heroes. All Might tells his old master the team needs him because All For One is sure to get involved. Gran Torino's team goes with the Police Force to the villain's secret hideout. 
All Might breaks into the hideout and Kamui Woods restrains all the villains with his Lacquered Chain Prison move. Dabi tries to use his fire to burn the wooden bonds, but Gran Torino swoops in a delivers a swift kick to Dabi's head. This knocks the villain unconscious and All Might commends Gran Torino and Kamui Woods. Gran Torino tells all the villains to take their arrest very seriously and to cooperate.
Tomura Shigaraki orders Kurogiri to open a Warp-Gate. Edgeshot suddenly uses Foldabody to pierce the villain's body and makes him fall unconscious. Gran Torino reminds the villains to take things seriously. He reveals identities of most of the villains. The Police discovered their true identities without much information to go on. Gran Torino then asks Tomura where All For One is hiding.
Suddenly, multiple Nomu get warped into the hideout through a black liquid. Gran Torino tells Edgeshot to stop Kurogiri, but the Ninja Hero confirms that it's not Kurogiri's doing. Katsuki Bakugo is swallowed by the black liquid. Gran Torino fights off the Nomu and asks All Might what they have to do now. All Might replies that he fears All For One has already maved his move.
The black liquid begins to consume the bonded villains. Gran Torino yells for the heroes to stop them, but they disappear within seconds. More Nomu attack and Gran Torino tells All Might to counter. All Might sends the Nomu flying with Oklahoma Smash and then decides to go after All For One at the Nomu Factory.
Izuku and his friends rescue Katsuki from the battlefield while All Might fights All for One. Magne, Mr. Compress, Spinner, and Twice try to pursue them using Magnetism. Mt. Lady foils their attempt to send Compress after the students. Gran Torino arrives and knocks the rest of them out before they can send someone else flying after Katsuki. All Might says Gran Torino is too slow, to which the old man replies his old protege is simply too fast. Gran Torino scolds All Might for Izuku arriving on the battlefield and says that they're too much alike in a bad way.
All For One recognizes Gran Torino as Nana Shimura's friend. Gran Torino prepares to deal with Tomura and Himiko himself. He attacks Tomura, but All For One sends all the villains through a Warp-Gate by forcibly activating Kurogiri and Magne's Quirks. All Might attacks All For One. The villain uses Warping to put Gran Torino in the path of All Might's punch. Then All For One uses Impact Recoil to invert the shockwave created by the punch, sending All Might back. Gran Torino is injured and All Might apologizes for accidentally striking his teacher.
Gran Torino remains momentarily unconscious while suspended in the black liquid. All For One attempts to kill him with an enhanced Air Cannon. All Might grabs Gran Torino and pulls him out of the way before countering with a Detroit Smash. Gran Torino wakes up from the shock when All Might crushes All For One's mask. He notices All Might beginning to revert back to his weak form.
All For One taunts All Might about his master and All Might tries to strike him out of rage. All For One takes advantage and blasts All Might towards a helicopter. Gran Torino flies in and grabs All Might just in time. He reminds his former student not to get distracted like the initial fight between the two six years ago. The old man notes that All For One's Quirks and fighting style have changed completely. He advises All Might to use his wits rather than brute force. Gran Torino tells him he needs to enter crisis mode and asks if the Symbol of Peace can still fight.
All For One attacks with a maxed out Air Cannon that blows Gran Torino away. All Might takes the brunt of the attack to save a nearby civilian. He's weakened so much that he reverts completely back to his weak form, revealing it to the world.  Gran Torino notices All Might can only make his arm muscular and worries that he can't take any more of the villain's attacks. Endeavor and Edgeshot arrive to assist All Might, having already dealt with the remaining Nomu. Kamui Woods comes to help rescue the defeated heroes. Each of them encourages All Might to win.
Gran Torino recalls talking to Nana about Toshinori for the first time. She told him that Toshinori was kind of crazy because he wanted to become a pillar for peace and justice for society. Gran Torino believes that Toshinori is this pillar and everyone's number one hero. He silently tells All Might that the entire world cheers him on, including his students who aspire to be just like him.
All For One blows the other heroes away and then activates his ultimate combination of Quirks. All Might uses advice from both his masters to gain the upper hand on his nemesis. All Might is able to win by using the remaining embers of One For All in his attack: United States of Smash! The world watches and praises All Might for winning. He enters his muscular form one last time and Edgeshot says he shouldn't push himself. Gran Torino tells Edgeshot to leave All Might alone because the Symbol of Peace still has one last job to do.
Following All For One's arrest, Gran Torino and Naomasa visit All Might in the hospital. All Might confirms he cannot use One For All anymore and says he still desires to go after Tomura. Gran Torino says he cannot because now he knows that Tomura is Nana's grandson.
Naomasa inquires about Nana's inleage and Gran Torino explains that her husband was killed and she put her only child into foster care to protect them. Nana also requested that Gran Torino and All Might have no communication with them. Before leaving, Gran Torino tells All Might that he and the police force will handle pursuing the League of Villains. All Might's job now is to raise Izuku to become the new Symbol of Peace.
On the phone with Izuku Midoriya, being told about his intentions for an internship, Izuku asks if he is accepting anyone. Gran Torino tells him he has got enough on his plate to be taken of him but realizes no one else reached out to him at the U.A. Sports Festival. Izuku says he will do whatever it takes to be the Number One hero. Gran Torino then suggests he should speak with All Might. Izuku mentions Shota Aizawa told him to use the connections he made at the festival. At the End of the Call Gran Torino says All Might should be able to introduce him to a number of people, such as his former sidekick.
During the meeting when Centipeder mentions observing Overhaul and Twice making contact. Gran Torino mentions about it being related to the League of Villains, and that is why he and Naomasa Tsukauchi were called. Telling Izuku to listen, he admits he never thought it would come to him being dragged into quite an annoying situation. When Sir Nighteye mentions about Kai's daughter being covered in bandages, Gran Torino asks what they expected of the world of superheroes as it all takes is the thought and anyone can do anything.
When tensions between Sir Nighteye and Fat Gum begin to flare, Gran Torino says they must not lose their cool. If they make a misstep trying to capture them it could end up fanning the flames. Similar to how Stain's arrest gave League of Villains a PR boost - This time with Chisaki trying to release a Quirk destroying the weapon. It is possible that those motives are also at play. Shortly afterward, Sir Nighteye thanks everyone for their cooperation and the meeting ends. Standing next to Sir Nighteye, the latter asks him if the reason he directed Izuku towards him was to mediate his relationship with All Might. Gran Torino denies it and says it was just a thought that crossed his mind and asks what he thinks. Sir Nighteye says they are both similar including the madness in their core that he could never understand.
During the battle between the Pro Heroes and the Eight Bullets, Gran Torino and Naomasa Tsukauchi have managed to track down and find Kurogiri with the help of eyewitness reports and have subdued him. Gran Torino tells Kurogiri that if they arrest him, the League of Villains will fall apart. As Naomasa warns Gran Torino about Kurogiri's Quirk, Kurogiri asks Gran Torino if he knows of the rumors about a "ruffian" appearing on the mountain where they are at and the reason he is here is to speak to the "ruffian". Gran Torino would prefer Kurogiri to tell him all about it behind bars. Suddenly, a huge noise reverberates in the area which catches Gran Torino's attention. An enormous beast that towers over trees appears with a boombox around his neck, which surprises Gran Torino. Kurogiri states that the ruffian is one of All For One's faithful servants: Gigantomachia.
He and Tsukauchi are seriously injured and the task force suffer severe casualties at the hands of Gigantomachia, but they managed to take Kurogiri into police custody, at the cost of allowing Gigantomachia to escape, after having completely destroyed a large portion of the mountain. Later, Gran Torino phone All Might to inform him they captured Kurogiri, and to warn him about the threat posed by Gigantomachia, which he defines as a walking disaster.
Because Izuku had a dream where the vestiges of One For All communicated with him, Toshinori calls Gran Torino to ask him if his master Nana ever mentioned anything to him about similar visions.
Hours later, All Might recieves a call and walks over to a private area to speak to Gran Torino. Gran Torino tells him about Nana's dream about One For All, The old hero mentions that around the time when Nana inherited One For All, she had dream about a strange figure shrouded in dark fog telling her "It’s not time yet…"
Gran Torino continues to collaborate with detective Naomasa to obtain information that can help locate the whereabouts of Shigaraki Tomura and the other members of the League of Villains. They try to interrogate Kurogiri, but he shows no sign of confessing or disclosing anything about the League of Villains, rather talking about useless stuff only to be completely unresponsive when bringing up the League of Villains.
When investigating about his person, they discover that he is a Nomu, and his Quirk is a combination of multiple Quirk factors combined into something new, which made it so complex that it took them time to notice. They also discover that the root factor bears a striking resemblance to a Quirk used by Oboro Shirakumo, an old high-school friend of Shota Aizawa and Hizashi Yamada who died in a villain attack.
Since Nomu with their own conscience seem to retain bits of personality from their past lives, Gran Torino and Naomasa decide to ask Shota and Hizashi for help, hoping that their connections with Oboro may cause some reaction in Kurogiri, so they can get information for the investigation. Once Shota and Hizashi arrive at Tartarus, they explain the whole situation and the reason why they are there. Shota says that if they think they can save him through their 'bonds' they're getting their hopes up. Gran Torino replies that with a strong enough basis, miracles can be possible.
Shota and Hizashi enter the visiting room where they start talking to Kurogiri. Their words finally manage to take effect, and the black fog that forms Kurogiri's head scatters enough to show the shadowed face of Oboro Shirakumo. Oboro manages to say "hospital" before the fog hides his face again and Kurogiri falls unconscious.
Naomasa and Gran Torino thanks to them because they have made progress. Gran Torino apologizes for having forced them to reopen an old wound but something's bound to come of it. When Shota wonders why All For One creates the Nomu, Gran Torino recalls a metaphor told to by the villain, in which he compares the creation of the Nomu as producing wine: trampling grapes (people) to extract the juice (Quirks), and letting them ferment into wine (Nomu).
That word turns out to be an essential clue in the investigation being carried out by the Hero Public Safety Commission, who have Hawks infiltrated the Paranormal Liberation Front, the new villain organization that emerged from the union of the League of Villains and the Meta Liberation Army. For several months, Hawks leaks essential information aboute the PLF to the Commission: hideouts, members, plans, strength, structure, etc ..... and most importantly, the location of its leader, Tomura Shigaraki, in the Jaku General Hospital.
By the end of March, the Commission organized the hospital raid and the attack on the PLF headquarter with a large force of Pro Heroes. Gran Torino joins the team of heroes led by Endeavor in charge of storming the hospital. Before the raid started, Gran Torino participates with other heroes in a meeting where Detective Naomasa informs them that Kyudai Garaki, the current board chairman of Jaku General Hospital, is Tomura's ally, and that there is a section of the hospital that only he can access.
The raid begins. As one group of heroes evacuate hospital visitors, workers and patients, the other pursues Garaki to his secret laboratory as they face the nomu he releases to keep the heroes busy as he tries to wake up Tomura. Despite everything, the heroes defeat their creatures and assault his laboratory, destroying the capsule where Tomura was in the process and preventing him from waking up. Facing his defeat, a heartbroken Dr. Garaki declares that the Devil's Dream has finally ended.
However, the unthinkable occurs. Tomura Shigaraki awakens, despite having previously verified that his heart was not beating. He immediately uses his decay, which spreads rapidly through the laboratory, disintegrating everything in its path. Gran Torino charges in and quickly grabs Present Mic and Kyudai Garaki as he notices how Tomura's Decay was spreading to things he hadn't even touched, which is something they didn't know about. He yells to the Heroes to escape and not touch the cracks unless they want to die.
The heroes try to flee the laboratory, but only a few manage to escape before the decay destroys the entire hospital and expands to the nearby city.
After destroying the hospital, Tomura heads to the nearby city for Deku, as something inside urges him to get the One For All. Izuku decides to get as far away from the city as possible to not to put people in danger, being followed by Katsuki. Tomura finally catches up with Deku and Katsuki, ready to attack them.
All of a sudden, Gran Torino rushes in and grabs the two of them before flying in the opposite direction. He explains he heard Tomura's intentions he knew something dangerous was going to happen. He also tells Deku that facing Tomura now is a bad idea because his Decay is so powerful merely touching the falling debris will ensure instant death. Deku tries to protest but Gran Torino reminds him that there are other Heroes that can still fight, and as powerful as he has become, he can't beat all of them.