During third grade, a new transfer student was not able to give a proper self-introduction, and because of his social awkwardness, he was not able to make any friends. Mirio approached this student, Tamaki Amajiki after realizing that Tamaki was trying to say that he likes Heroes; it was this recognition that began the friendship between Tamaki and Mirio. In middle school during a Quirk Education class, Tamaki tried manifesting something but only managed to manifest a tiny plant on his hand due to his nervousness. Meanwhile, Mirio tried phasing through a wall but only managed to phase his hands through it. Some of Mirio's classmates laughed at his struggles, but he was not disheartened at all and resolved to pull off his phasing properly next time. While walking home, Tamaki praised Mirio's optimism while lamenting his own lack of self-esteem. Mirio cheered Tamaki up and said that he would shine brightly like the sun and the reason Mirio could give it his all was due to Tamaki being there for him. Although he was nervous, Mirio praised Tamaki's resolve to never give up which inspired him to never give up either. Despite Tamaki thinking of himself as a boring person with low self-esteem, Mirio always thought of Tamaki as a talented person who was cool and super fun and sunny to be around with. 
After middle school, Mirio and Tamaki entered U.A. According to Nejire Hado, Mirio once struggled a lot at U.A. High School (even showcasing poor results in an edition of the Sports Festival); however, he would turn his situation around by devoting himself as an intern under the hero Sir Nighteye and having his abilities cultivated during his internship, eventually becoming a major contender for the number one rank, above several pro heroes.
Upon introducing themselves, he mentions internships about Class 1-A and challenges them to a fight. Inside Gym Gamma after Class 1-A attacks first, he defeats the long-range quirk users with a single strike.
Now targeting close-ranged quirk users, he listens as Izuku has his classmates form a strategy and he runs towards them, intrigued by their next move. He phases through the ground and appears behind Izuku who anticipated this and he is surprised by the latter's deductions. Appearing behind them and defeating the students one after another. When the fight finishes, Mirio asks if they found his quirk strong and then reveals its functions. Explaining how his internship helped with his quirk, he says it's an opportunity they can't afford to miss out on. After leaving the gym, Tamaki Amajiki asks him if anyone caught his eye and Mirio mentions Izuku Midoriya's performance.
Seated inside the restroom next to Izuku Midoriya, All Might asks his own opinion on Izuku working under Sir Nighteye. All Might mentions he would be unable to face Sir Nighteye himself. Mirio asks Izuku what kind of hero he wants to be. When Izuku reveals his goal of wanting to be the greatest hero, he claims he has no reason to refuse introducing Izuku to Sir Nighteye. Later that day he takes Izuku to the area of Sir Nighteye's offices and explains he has very strict standards. Proceeding to guide him to Sir Nighteye's office, he offers him pieces of advice along the way.
Freeing Bubble Girl from a shackled wall, she asks what Izuku is to him, Mirio mentions that he's an underclassman he's taking care of. He observes Izuku and notes that although he wasn't able to make Sir Nighteye laugh, he can't argue with the result and that it got his foot through the door. When Sir Nighteye challenges Izuku to a test and asks Mirio and Bubble Girl, she asks him if he had to undertake something similar. He reveals Sir Nighteye personally chose him and he didn't. As Bubble Girl reveals her jealousy, Mirio begins to think about Izuku's test against Sir Nighteye. 
When they both re-enter Nighteye's office Mirio asks if it's over as it got pretty loud. Nighteye tells Mirio that Izuku is hired. The next day stood outside with Sir Nighteye, Bubble girl, and Izuku, they inform Izuku of their secret investigation. Both Mirio and Izuku begin to patrol the area until they encounter a girl with Kai Chisaki behind her.
Upon encountering Chisaki, Mirio throws Izuku his mask and says they should be the ones to say sorry. Chisaki mentions it is the first time he's seen them, and asks if they are new, after which Mirio tells him that they are. When Chisaki asks what office they're with, Mirio tells them they're students being allowed to patrol for experience, knowing he can't reveal that he is with Sir Nighteye. As Mirio asks for them to leave, Izuku begins to start asking Chisaki about the girl. As they follow Chisaki into an alley for further questioning, the girl returns to his side and they bid farewell to the heroes. Izuku goes to follow them but Mirio stops him. Asking if he noticed the killer's aura around Chisaki, he says to respect Sir Nighteye's orders.
Meeting up with Sir Nighteye, he apologizes for having an incident. Sir Nighteye says it was an error on his part, and if he'd seen it he could have prevented it. Afterwards, Mirio tells Nighteye about Chisaki's daughter. Izuku mentions wanting to save her. Sir Nighteye tells them both the cleverest villains hide in the shadows and now is a good time for them to learn. 
Throughout the meeting when Izuku and Gran Torino begin talking, Mirio questions about them knowing each other. When Fatgum mentions Tamaki Amajiki was shot by a bullet that can destroy quirks a concerned Mirio asks if he is okay. When Sir Nighteye mentions Chisaki's quirk along with bullets that can destroy quirks, Mirio becomes terrified. When they come to the conclusion that Eri's body is being used to create bullets, both Izuku and Mirio vow to protect her.
When the meeting ends Mirio returns to U.A. High School sitting at a table surrounded by his fellow students with his head sunk low, Tamaki asks him to lift his head. Nejire Hado asks Mirio if he's feeling down or regretful. She asks if he knows he can do something about it. Mirio says yes he does.
Days later, late at night the Big 3 gather after receiving a message containing information, 'on the day carry out the plan to raid Shie Hassaikai'. Hours later, everyone gathers together and Mirio becomes fired up. Sometime later, after changing into his hero costume alongside everyone else they gather outside the home of Shie Hassaikai. 
Inside the house they reach the underground and believe to have found themselves at a dead end. Mirio prepares to go ahead to look and phases through the room. Seeing a corridor like Sir Nighteye foresaw Mirio returns and tells him a wall is blocking their way. After Izuku and Kirishima knock down the wall the floor starts to warp. Tamaki notes that if the pathway keeps changing they'll never reach their goal. Mirio tells him that won't happen, and that it's all a stopgap measure. He asserts that even if the passage is warped as long as he knows the right direction he can get there and phases into the floor. 
Mimic causes the floor to open up beneath the Heroes, causing them to fall down a single floor. The heroes are in a hall; three figures appear from the dust, revealing themselves to be three members of the Eight Expendables.
Mirio appears before Chisaki and attacks simultaneously. Mirio strikes Chisaki with a backhanded smack, which Chisaki avoids but Mirio still manages to leave a mark on Chisaki's face while kicking Chronostasis. Mirio avoids hitting Eri thanks to his Permeation and strikes Chronostasis down with his kick. Chronostasis falls back as Mirio grabs hold of Eri. Eri tells Mirio to run away otherwise Chisaki will kill him. However, Mirio promises to never let Eri be sad again and tells her that everything is fine because he is her Hero.
Chisaki begins wiping off the mark Mirio left on his face as he threatens Eri to return to him because she was born to destroy people, otherwise Mirio will be murdered. Before Eri can speak, Mirio tells her to not listen to Chisaki's words. When Chisaki mocks Eri for her existence being a curse, much to Mirio's chagrin who wonders why Chisaki is saying such harsh words to his own daughter. However, as he takes off his glove, Chisaki reveals that Eri is not his daughter. Chisaki places his hand on the ground and deconstructs it using his Quirk. 
Chisaki reconstructs the crumbled ground into massive spikes that attack Mirio, but Mirio quickly phases his body while holding up Eri with his arms, protecting both of them. Mirio is surprised at Chisaki's quick speed as well as his lack of regard for Eri while Chisaki himself praises Mirio for being more than just his Quirk. Chisaki launches more spikes at Mirio, uncaring at Eri's safety since he can just "repair" her easily and reveals to Mirio that Eri has experienced it firsthand; Chisaki's words enrage Mirio. He continues avoiding the spikes while Chisaki asks him about Eri's safety in his hands since if she gets hurt she cannot be healed immediately and that having her is making it troublesome for him as he cannot run away using his Permeation while carrying Eri. Due to him reconstructing the area with spikes, Chisaki has managed to cut off Mirio's escape route. Chronostasis has awakened and has a gun in his hand that is loaded with Quirk-disabling bullets. Mirio is surprised that Chronostasis is still awake after being hit by him and assumes that Deidoro Sakaki's Quirk made his physical attacks weaker at the time. Chisaki orders Chronostasis to aim at Mirio's arms while he destroys the spikes so that Mirio loses his ground. Chisaki deconstructs the ground, causing Mirio to be in mid-air. Mirio conceals himself and Eri using his cape; Chronostasis fires at Mirio but misses due to his target being concealed by the cape.
Mirio tries to appear before Chronostasis but Chisaki anticipated this strategy and begins reconstructing the ground beneath his second in command. Mirio does indeed appear before Chronostasis, using his Quirk's instant transportation to do so. Mirio's shoulder knocks Chronostasis's arm which causes him to drop the gun and is about to hit Chronostasis with a punch, but Chisaki launches Chronostasis away from Mirio by reconstructing a platform which saves Chronostasis, who apologizes to Chisaki. Seeing that Mirio is stronger than he thought, Chisaki decides to break Mirio's morale by killing Eri. Before he can do so, Mirio uses his instant transportation to appear behind Chisaki. Chisaki realizes this and tries blocking Mirio with his hand, but Mirio phases his arm through Chisaki and punches him in the face.
Chisaki lands on the ground as Mirio states that his cape is for bundling up Eri who is in terrible pain. Chronostasis gets up and tries getting the gun, only for Mirio to appear before him and knock him back down. Mirio praises Chisaki for his abilities being far above that of the average yakuza but tells him that he is far stronger, using his instant teleportation to appear before Chisaki and lands another punch on him. Eri watches the fight and is surprised at not only Mirio's strength but the lengths he is willing to go for her. Mirio swears to never let Chisaki lay another finger on Eri and promises to take them both down, declaring that he has them in checkmate.
Chisaki becomes enraged at Mirio calling out his name since he abandoned it long ago. Years ago during childhood, the boss of the Eight Precepts of Death took Chisaki in since he had nowhere he could call home and asked for Chisaki's name, to which he states that his full name is Kai Chisaki. Several years later, the boss of the Eight Precepts of Death lamented about the destruction of a yakuza group; Chisaki told the boss about the plan he had discussed with him before, but the boss did not take heed of it because falling off from the part of humanity would cause his demise as a gangster since people would not follow brutes with no hearts.
Mirio immediately jumps towards Eri. Mirio was able to tell that Eri was endangered because she wouldn't scream or cry whenever murderous intent was directed at her and she would simply close her eyes, be silent and grit her teeth; this was Eri's habit that was engraved in her after living in pain and fear for a long time.
Mirio reaches Eri while Shin fires the Quirk destroying bullet. Eri opens her eyes and sees a smiling Mirio who is about to be hit by the Quirk destroying bullet. Mirio thinks about his childhood where his family's Quirk made it difficult for them to become Heroes, which is why his father gave up on becoming a Hero. However, Mirio refused to waver and throughout his life, he worked extremely hard to hone his Permeation; eventually, Sir Nighteye saw potential within Mirio and made him his sidekick. Under Sir Nighteye's guidance, Mirio was able to turn his Permeation into a full combat Quirk; during this time Mirio along with Tamaki and Nejire began being acknowledged as the Big Three.
Mirio covers Eri as the Quirk destroying bullet is shot into his body which begins destroying Mirio's Permeation. Chisaki states that due to Quirks people are entitled to dream and begin thinking that they become people that matter, to which Chisaki believes that these people are sick in the head. As Shin celebrates his triumph, Chisaki laughs at Mirio's efforts and mocks Mirio for smiling because everything he has worked so hard to cultivate has been reduced to naught. As Chisaki is about to deconstruct the ground, Chronostasis is thrown at him, courtesy of Mirio's kick. Mirio appears before Chisaki and punches his arm. Mirio tells Chisaki that all his hard work isn't in vain because he will still be Lemillion. Chisaki tries attacking Mirio but he, while quirkless, fights him back for five minutes all in order to protect Eri from Chisaki.
In the aftermath of the Yakuza operation. When Nighteye begins to succumb to his wounds, Mirio runs into his room and begged his mentor not to leave as he still had much to learn from him. Nighteye comforted Mirio by using his Quirk one final time to tell him he'd be a great hero in the future.
Much later, Izuku visits Mirio who is doing exercises on his bed, surprisingly in a cheerful mood despite all that has happened. Mirio is glad that he is in top shape and he knows that he has lost his Quirk along with Sir Nighteye and has no right to be cheerful. However, Mirio has decided not to be sad because Sir Nighteye was smiling all the time while talking to him and will respect his teachings by also smiling since he is going to be an outstanding Hero in the future and having a glum face will make Eri sad.
When Izuku hypothetically asks Mirio if he could give Mirio his Quirk, would Mirio accept it. Surprisingly, Mirio replies that he would refuse Izuku's offer, telling Izuku that by accepting his Quirk, Izuku would suffer hardship. Although he does not understand why Izuku feels dejected, Mirio thanks Izuku for doing a great job. Mirio feels positive about the situation because when he spoke with Aizawa about when Eri gets control of her Quirk in the future, she will be able to restore him to a state where he had his Quirk. Mirio asks Izuku to smile; Mirio's optimism convinces Izuku to accept the reality. Izuku tells Mirio that he will be waiting for him to return to U.A. while he is taking a temporary leave of absence.
Visiting Eri in hospital, alongside Izuku Midoriya. While Izuku apologizes to Eri for being unable to see her earlier Mirio gifts her a fruit basket. Eri apologizes to them for all the trouble for her sake. When Eri starts feeling guilty for Mirio losing his Quirk, Mirio tells Eri that she is not to blame and everyone is glad that she is safe. Suddenly, Izuku has an idea and asks Shota Aizawa if Eri can come to the U.A. Cultural Festival. Mirio explains to Eri that the festival is an occasion that U.A. High School hosts lots of events on display where people can enjoy themselves while tasting delicious food.
At the Festival, Mirio takes Eri to the show which Class 1A has set up and he holds her up so she could see it. When Eri sees Izuku performs, a happy Mirio watches with tears in his eyes as the child laughs for the first time. 
Later on, Mirio and Aizawa inform Izuku and Class 1A that Eri will be taken in by the school. Mirio explained the growth of her horn and states they need to monitor her so it doesn't get out of control. Tamaki Amajiki pats him on the shoulder and mentions if Eri's abilities become stable and she gains control, Mirio's quirk could possibly return to him. Mirio laughs at the idea by stating it would be neat as he smiled at his friend.