When he was a small child, Mirio accidentally fell into a river but was saved by a Pro Hero. This event marked his decision to also become a hero to save others. Although his father considered that, due to his Permeation Quirk, it would be a difficult goal to achieve, Mirio did not lose heart and spent the rest of his years training to improve control of his Quirk and fulfill his dream.
During third grade, he met Tamaki Amajiki, a new transfer student who was unable to give a proper self-introduction because of his social awkwardness and as a result, was not able to make any friends. Mirio approached him after realizing that Tamaki was trying to say that he likes Pro 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 with his Manifest Quirk 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 and resolved to properly pull off his phasing 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. High School. According to Nejire Hado, Mirio struggled a lot at U.A. (even showcasing poor results in an edition of the U.A. Sports Festival); however, he would turn his situation around by devoting himself as an intern under the hero Sir Nighteye, and cultivate his abilities to eventually become a major contender for the number one rank, above several Pro Heroes.
While Izuku Midoriya is taking out the trash Mirio appears before him inside a wall and gives Izuku instructions on where to put the trash, and then recedes into the wall. Mirio then appears in the ground, surprising Izuku again. Izuku asks him what he is, to which Mirio replies he will know about him soon enough and encourages him to keep his spirits up. As it leaves, Mirio tells Izuku about a rumor flying around and asks him to keep his wits about him. He falls into the ground before he can finish his sentence.
Three days later, alongside Tamaki Amajiki and Nejire Hado, they are introduced to Class 1-A by Shota Aizawa as The Big 3. After his teammates introduced themselves to the students, without making a good impression due to their extravagant personalities. Mirio steps in and tries to break the ice with a joke. No one in Class 1-A understands his joke and they start to wonder why such esteemed students have such strange personalities. Mirio insists that the best way for first-year students to learn about work studies is to fight him.
Mirio sees that Class 1-A does not understand what is going on and tells them that he understands that feeling. Mirio comments that Class 1-A is energetic due to them receiving their Provisional Hero Licenses as first years. Seeing that his introduction has fallen flat, Mirio offers Class 1-A the chance to team up together and fight him, much to Class 1-A's surprise. Mirio decided to challenge Class 1-A because having them witness their experiences personally would be better than a simple explanation. Shota tells Mirio to do as he pleases.
Everyone moves to Gym Gamma where Class 1-A (save for Shoto and Katsuki) all at once against the lone Mirio. Hanta Sero asks Mirio if he really wants to fight all of Class 1-A at the same time, to which Mirio replies that he is serious. Then Eijiro asks Mirio if he sees them as nothing but small fry, to which Mirio replies that he does. Mirio tells Class 1-A to attack him whenever, wherever they want and asks who wants to go first. Izuku accepts the challenge and goes first.
Class 1-A's close combat team tries to surround Mirio, but everyone is taken by surprise when Mirio's clothes suddenly fall off his body. Izuku tries to exploit the openings in his opponent's defense but his kicks go clean through Mirio's physical form. The other students launch an ambush and Mirio swiftly appears behind everyone. The students believe he warped using his Quirk and Mirio attacks the long-range fighters. In a few minutes, he defeats the long-range Quirk users with a single strike. Shota reveals that Mirio is the closest hero to being No. 1 while even including the Pro Heroes.
Now targeting close-ranged Quirk users, he listens as Izuku Midoriya 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 anticipates this, and he is surprised by the latter's deductions. Despite this, Mirio easily overcomes Izuku with one of his super moves. He then appears behind and defeats 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 asks him if anyone caught his eye and Mirio mentions Izuku Midoriya's performance.
Later on, standing alongside Izuku they visit the restroom where All Might invites them to come in. Seated inside the restroom next to Izuku, 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 to introduce 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 words 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, and that makes Mirio happy. The next day he stands outside with Sir Nighteye, Bubble girl and Izuku, and 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 Kai, Mirio throws Izuku his mask and says they should be the ones to say sorry. Kai mentions it is the first time he's seen them and asks if they are new, and Mirio tells him that they are. When Kai 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 Kai about the girl. As they follow Kai 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 Kai, 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. Afterward, Mirio tells Nighteye about Kai'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. 
A few days later, Mirio attends the meeting regarding Shie Hassaikai in Sir Nighteye's offices. Throughout the meeting when Izuku and Gran Torino begin talking, Mirio questions about them knowing each other. When Fat Gum mentions Tamaki was shot by a bullet that can destroy Quirks, a concerned Mirio asks if he is okay. When Sir Nighteye mentions Kai's Quirk along with bullets that can destroy Quirks, Mirio becomes terrified. He and Izuku understand at that time what Kai was really doing to the girl: he deconstructed and recomposed her body constantly with his Quirk to get her blood to create the bullets. Upon understanding that, both Mirio and IZuku are devastated, knowing that they could have saved Eri when they first met her. Izuku and Mirio vow to protect her.
When the meeting ends, The U.A. students meet in the back, saddened by the state of the girl. Mirio and Izuku are sitting at a table surrounded by their fellow classmates with their head sunk low, after explaining their encounter with Overhaul and Eri. Shota appears and his words get everyone's spirits up. Tamaki asks Mirio to lift his head. Nejire asks him if he's feeling down or regretful. She asks if he knows he can do something about it, and Mirio says that he can.
For several days, students wait patiently to receive any news of Eri's whereabouts. One day, 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. When everything is ready, the group raids the house.
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 Eijiro Kirishima knock down the wall, Mimicry uses his Quirk to alter the underground labyrinth. 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.
Thanks to his Quirk, Mirio manages to catch up with Chronostasis and Kai. Mirio tells them he is there to rescue Eri, but Kai is adamant that Eri doesn't want to be in Mirio's custody and mocks Mirio for his futile heroics, to which Mirio replies that he came to rectify his mistake.
Mirio charges towards Kai, but suddenly his sense of balance is affected and he has a hard time standing. Without knowing it, in that place, there are also two members of the Eight Bullets: Deidoro Sakaki, whose Sloshed Quirk is making him unbalanced, and Shin Nemoto, who fires his gun at Mirio. However, the bullets phase through him. Using his Confession Quirk, Shin asks Mirio about his Quirk, which forces Mirio to reveal his Quirk's ability.
Understanding the ability of Mirio's Quirk, Shin tries to break his spirit by forcing him to answer his questions, which reveals Mirio's insecurities and aspects of his personality he did not know. Especially mortifying for Mirio is when Shin tells him that compassion is unneeded for progress to smoothly flow, and makes Mirio recognize that this is the reason he did not help Eri the first time. Shin questions Mirio again about hiss motives for saving Eri, wondering if he only wants to save her to make himself feel better, what affects Mirio mentally.
The combination of Shin and Deirodo’s Quirks make Mirio stops in his tracks, dizzy and dejected. Seeing the opportunity to end him, Shin fires his bullets and Deidoro throws his knives at Mirio. Suddenly, Mirio uses Phantom Menace, a move in which he phases himself and quickly strikes down his opponents, knocking down both Shin and Deidoro.
Mirio agrees with Shin that he let Eri go back into despair and already knew that he carried a sin for doing so, but now he is there to save Eri, so she doesn't have to suffer again. Mirio cannot stand to see Eri without a smile on her face and finds that to be unforgivable. Mirio quickly strikes Deidoro and Shin again and charges towards Overhaul.
Mirio appears before Overhaul and attacks him. Mirio strikes him with a backhanded smack, which Overhaul avoids but Mirio still manages to leave a mark on his 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 Overhaul 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.
Overhaul 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 Overhaul's words. When Overhaul mocks Eri for her existence being a curse, much to Mirio's chagrin who wonders why Overhaul is saying such harsh words to his own daughter. However, as he takes off his glove, Overhaul reveals that Eri is not his daughter. He places his hand on the ground and deconstructs it using his Quirk.
Overhaul 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 Overhaul's quick speed as well as his lack of regard for Eri while Overhaul himself praises Mirio for being more than just his Quirk.
Overhaul 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; Overhaul's words enrage Mirio. He continues avoiding the spikes while Overhaul 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, Overhaul 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 Sloshed made his physical attacks weaker at the time. Overhaul orders Chronostasis to aim at Mirio's arms while he destroys the spikes so that Mirio loses his ground. Overhaul 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 Overhaul anticipated this strategy and begins reconstructing the ground beneath his second in command. Mirio does indeed appear before Chronostasis, instead of 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 Overhaul launches Chronostasis away from Mirio by reconstructing a platform which saves Chronostasis, who apologizes to Overhaul.
Seeing that Mirio is stronger than he thought, Overhaul decides to break Mirio's morale by killing Eri. Before he can do so, Mirio uses his instant transportation to appear behind Overhaul. Overhaul realizes this and tries blocking Mirio with his hand, but Mirio phases his arm through Overhaul and punches him in the face.
Overhaul 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 Overhaul 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 Overhaul 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 Overhaul lay another finger on Eri and promises to take them both down, declaring that he has them in checkmate.
Overhaul becomes enraged at Mirio calling out his name since he abandoned it long ago. Then, he sees a wounded Shin Nemoto. Despite his defeat at the hands of Mirio, because of his undying devotion to Kai, Nemoto manages to drag himself to the place where the fight was taking place. Kai sees Shin as well as Mirio. Kai immediately throws a case of Quirk-disabling bullets at him. Shin opens the case and loads one of the bullets in his gun and Kai orders him to open fire.
Shin knows how valuable bullets are, so he cannot waste a single shot. He knows he can't shoot Mirio because he would avoid the bullet with his Quirk, but realizes that Mirio will do whatever it takes to protect Eri, so he aims his gun at her. Mirio immediately jumps towards the girl.
Mirio has reached 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 reaches Eri while Shin Nemoto fires the Quirk-destroying bullet. At the moment before the bullet hits, Mirio reaches her and gets her attention so she can see him giving her a reassuring smile that says everything is going to be okay. Mirio thinks about the whole process he had to work to be a hero: dealing with his complicated Quirk, years of training to master him, when he met Sir Nighteye and he helped him 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. Overhaul states that due to Quirks people are entitled to dream and begin thinking that they become people that matter, to which Overhaul believes that these people are sick in the head. As Shin celebrates his triumph, Overhaul 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 Overhaul is about to deconstruct the ground, Mirio kicks an unconscious Chronostasis thrown at him. Mirio appears before Overhaul and punches his arm. Mirio tells Overhaul that despite being Quirkless, all his hard work isn't in vain because he will still be Lemillion. Mirio just keeps laying into Overhaul using nothing but the physical conditioning and foresight he trained to help him master his Quirk in the first place. He ends up taking hits without the ability to phase through Overhaul's attacks, but he continues to fight and protect Eri for five whole minutes before getting badly wounded, buying just enough time for Izuku to smash his way into the room.
When Izuku, Shota, and Sir Nighteye reach the place, Izuku immediately runs towards Kai and delivers a One For All: Full Cowl punch to his arm. Shota follows Izuku while Nighteye runs towards Mirio and Eri. He is amazed that his pupil has not only managed to defeat three members of the Shie Hassaikai and gave his boss a hard time, but also managed to rescue Eri. Sir Nighteye grabs hold of Mirio and Eri. A proud Sir Nighteye praises and congratulates Mirio for everything that he has done.
Izuku and Shota charge at the injured Kai; who tries using his Quirk but to no avail as Shota has disabled it with his Erasure. Cornered, Kai orders Chronostasis to wake up. He uses his Quirk and Shota's shoulder is struck by a minute hand that is emanating from Chronostasis's head. Shota is forced to blink due to the cut from Chronostasis, which allows Kai to use his Quirk again, who constructs massive spikes from the ground, which stalls Izuku and Shota while drawing Shin Nemoto's unconscious body towards him. Kai refuses to allow his plans to be ruined by the Heroes and uses his Overhaul to fuse together with Shin, taking a monstrous new form, to everybody's horror.
Having fused with Shin, Kai mocks Mirio for wasting his time trying to become a Hero as it has been all for naught now that he has lost his Quirk and could have avoided this tragic fate had he simply not gotten involved. Izuku and Sir Nighteye are surprised that Mirio has lost his Quirk. Kai continues bashing Mirio for continuing his Heroics even after losing his Quirk as it has now dragged his peers into his mess.
Sir Nighteye orders Izuku to protect Mirio and Eri while he handles Kai. Izuku goes to Mirio and Eri's location with his enhanced speed. Izuku reaches Mirio and Eri and sees that they are fine. Izuku smashes open the wall which reveals the path he along with Shota and Sir Nighteye took to get here. Izuku decides that they should put distance between themselves and Kai. Izuku carries Mirio and Eri into the pathway; Eri apologizes while Mirio looks back to see Sir Nighteye.
While Sir Nighteye, and then Izuku, fight a merged Kai, in the hallway, the damaged and tired Mirio Togata wills himself onward to take Eri to safety. However, Mirio's body begins to give out and crying because he's failing to do as he promised and is unable to escort her out of the Yakuza hideout. Mirio tells Eri that she needs to hide and wait for the others to come and find her. Finally, Mirio can not stand the wounds and fatigue for longer, and falls unconscious for the rest of the battle, being subsequently picked up by his Tamaki Amajiki.
In the time he was unconscious, Sir Nighteye was mortally wounded by Kai, and was about to do the same to Deku as well, but thanks to Eri's Quirk, Rewind, Izuku was finally able to defeat the leader of Shie Hassaikai, although Shota had to intervene to cancel Eri's Rewind, which threatened to kill Deku. Finally, the heroes rise with victory, Shie Hassaikai is dismantled and his members sent to prison, Eri is rescued, and the wounded sent to a hospital to recover.
In the aftermath of the Yakuza operation, all heroes are cured and there is no fear for their lives, except for one: Sir Nighteye. The wounds he suffered in his fight against Kai are so severe that even Recovery Girl cannot heal him. In his death bed, All Might and Izuku can't help sobbing at his impending death.
When Nighteye begins to succumb to his wounds, Mirio runs into his room and begs his mentor not to leave as he still had much to learn from him. Sir Nighteye ponders about only taking Mirio as a vessel for One For All but overtime Mirio held such admiration for him and believed in him, eventually with Mirio becoming his most treasured student and is guilty of once thinking of Mirio as nothing more than a vessel.
As Mirio weeps, Nighteye raises his arm and comforts him, using his Quirk one final time to see Mirio's future, a future where he becomes an outstanding Hero. Sir Nighteye closes his eyes and smiles, telling everyone to keep smiling as society needs smiles and laughs to bring about a brighter future. Sir Nighteye passes away peacefully as All Might, Izuku, and Mirio begin to cry.
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.
Izuku hypothetically asks Mirio if he would accept his Quirk if he gave it to him. 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 Shota 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 and 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.
With the beginning of October, Mirio goes to Sir Nighteye's funeral, along with the internship group, All Might, and Eraser Head. Centipeder takes over Nighteye's Office, and he and Bubble Girl tell Mirio that he is welcome back anytime.
Visiting Eri in hospital, alongside Izuku. 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 if Eri can come to the U.A. School Festival.
When Eri asks what a festival is, Mirio explains to her that the U.A. School Festival is an occasion that U.A. High School hosts lots of events on display where people can enjoy themselves while tasting delicious food like candied apples, which catches Eri's attention. 
Days later, while students in class 1-A practice for their performance at the School Festival, they are visited by Eri and Mirio. Shota Aizawa states that they received permission from Nezu to bring Eri to the School Festival. Mirio states that he is showing Eri around U.A. and invites Izuku to join them, which he accepts. Izuku, Mirio, and Eri walk through the U.A., and see many students making their preparations for the School Festival: The Class 1-B, U.A.'s Development Studio, etc... Later, in Lunch Rush Cafeteria, Eri comments on everyone doing their best and she wishes to see how their hard work will turn out in the School Festival. Izuku and Mirio are happy that Eri is feeling excited and tell her the emotion she is experiencing.
The day of the festival, hours before it starts, Mirio Togata, Shota Aizawa, and Eri are riding in a taxi to U.A. During the riding, Mirio talks to Eri about the schedule of the performances and events, being the dance of Class 1-A at ten a.m. Mirio asks Eri if she is eager to see Izuku, to which she answers yes. Before the dance begins, the three go to visit class 1-A, where they are told that Izuku was not there at that time, that he had left the U.A. to run errands and for some reason was taking longer than he should (They did not know that Izuku had been fighting and defeated Gentle Criminal). Fortunately, Izuku arrives on time.
At the Festival, Mirio takes Eri to the show which Class 1-A has set up and he holds her up so she could see it. When Eri sees Izuku and Class 1-A's performance, she can't hide her joy and smiles brightly, representing that Overhaul's grip on Eri's psyche is broken at last. Seeing Eri beaming with excitement at the school festival makes Mirio cry tears of joy at finally sees her smiling and laughing for the first time, being able to let go of her tormented past.
After the dance, Mirio, Shota, Deku, and others accompany Eri to enjoy the rest of the festival's events until it ends, and she must return to the hospital.
As November comes to a close, Shota, the Big 3 and Eri visit Class 1-A rooms. Shota reveals to Izuku, Ochaco, Tsuyu, and Eijiro that Eri will be looked after by U.A since she doesn't have any relatives who can take care of her. Mirio explains that the source of Eri's Quirk, her horn, is growing back slowly which is another reason Eri was transferred to U.A. Shota can prevent her Quirk from going out of control; he will be monitoring and helping control her power in the empty room in the teacher's lounge. Mirio will also help him to take care of Eri and welcomes Class 1-A to visit her once in a while since they will all be busy.
Tamaki then 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.
Then, Shota tells his students to return to their dormitories.
The day after the Joint Training Battle, Shota asks Mirio and Izuku to accompany Eri. It turns out he wants Neito to copy Eri's Rewind Quirk, however, Shota does not want to leave her alone with Neito, in case his eccentric personality could affect her mental health negatively.
Mirio watches Neito attempt to copy Eri's Quirk, but while he gains her physical horn, he is incapable of using the Quirk itself because is an accumulation-type Quirk. Neito apologizes for not being able to help more, but Shota thanks him for trying. Shota explains to Mirio and Izuku that as Eri doesn't understand how to properly use her power, he hoped that Neito's Copy Quirk would help her and make it easier for her to learn it as well.
When Shota takes Eri to enjoy Christmas with class 1-A, Ejiro asks him if Mirio would come too. Shota replies that Mirio is celebrating Christmas with his classmates.
The Big 3 are taking care of Eri in her room when she suddenly starts crying, saying that her horn's started to feel unusual. While Mirio and Tamaki hurry off to look for Shota, Nejire stays with Eri, hugging and comforting her. Shota arrives and erases Eri's Quirk, gently patting her on the head and telling her not to worry.