Mirai Sasaki/Synopsis

History
Mirai Sasaki was a huge fan of All Might and wanted to be his sidekick, but All Might refused due to his policy of not taking anyone. However, Mirai was so persistent that All Might eventually let him be his sidekick; though it's unclear when they started working together, it was at least as far back during All Might's silver age. Mirai, under the hero name of "Sir Nighteye", took on a supporting role, being the brains of his operations, while All Might fought on the front lines. Because all Heroes are required by law to file detailed reports of their Hero activities, Sir Nighteye did all the file work for All Might. At some point during this period, Sir Nighteye learned about the secret of Toshinori Yagi and the true nature of One For All.

During their partnership in All Might's silver age, Sir Nighteye is seen working at his desk after All Might finishes 72 consecutive hours of hero work. He urges All Might to get some rest, when an emergency dispatch request comes from Osaka, Japan. Nighteye tells All Might to wait so he can ask if any other local heroes are available to call, but All Might wastes no time and bolts toward Osaka.

They got along well until six years ago, when they dissolved their partnership.

All Might had a fearsome and gruesome battle against All For One. While fighting him, All Might received critical injuries: his respiratory system was completely damaged, which set a limit on the usage of his Quirk. While All Might was hospitalized, Sir Nighteye used his Foresight on him to see his future, and what he saw was discouraging.

Sir Nighteye begged All Might to retire due to this, as he didn't want All Might to push himself to the brink of death. All Might refused to since the world was waiting for his return and he had to answer their pleas. Sir Nighteye begged All Might to retire and let his legend live on for the sake of the peace he desired. Nezu agreed with Sir Nighteye and told All Might that he could look for a One For All successor at U.A. High School, with Sir Nighteye agreeing that Nezu's idea was the best solution.

All Might asked who the Symbol of Peace would be until he found that successor. Even though All For One was gone, another villain like him would eventually appear. Sir Nighteye warned All Might that he would stop supporting him if he planned on continuing as a Hero. All Might noticed that Nighteye was "looking" at him with his Quirk. Sir Nighteye tried getting All Might to agree, saying that even if he retired, the next Number One Hero would eventually rise and although that "short while" would be difficult, they may be able to ward the villains off.

However, All Might still refused since that short while would be a period of fear. All Might noted that Nighteye's Foresight was never wrong, to which Sir Nighteye feared as he has seen what would happen to All Might at this rate. Sir Nighteye told All Might the truth regarding his future in order to coerce him into retirement to save him: In six to seven years, All Might will confront a villain and suffer a gruesome death.

After this dispute, they went their separate ways. Nighteye started his own hero career, opening his own Hero Office. However, Nighteye didn't seem to hold any resentment or ill-will against All Might after their separation and consequent break up, remaining a fan of his.

Years later, All Might met Izuku Midoriya and decided that he was the perfect candidate to be the new One For All's successor. Toshinori told Nighteye about this, but Nighteye was vehemently against Izuku becoming the successor, which further drove them apart. Toshinori and Nighteye argued over the phone regarding Izuku; Toshinori argued that Izuku was intent on saving others while Nighteye argued that intentions were not good enough since there were more capable candidates with the same intentions as Izuku.

However, Toshinori stated that Izuku was Quirkless, which made him a befitting candidate over the others. Toshinori said that after their heated debate over the succession of One For All, Nighteye took it upon himself to cultivate Mirio Togata, whom he believed to be the truly appropriate successor.

Shie Hassaikai Arc
Sir Nighteye was first mentioned by Mirio and Gran Torino as a potential mentor for Izuku. He was first seen working on his computer in a dark room and being informed by Bubble Girl about the meeting between the League of Villains and Overhaul, the latter who was under investigation by Sir's organization.

He meets Izuku and notices him doing an imitation of All Might and he corrects him on the face. The two speak of the former hero's legacy and Nighteye is impressed by his knowledge but doesn't see any benefit to hiring him. After Izuku protests, Nighteye says he would only end up mentoring Izuku if he passes his test.

Nighteye explains that all Izuku needs to do is to get the stamp out of Sir Nighteye's hand and stamp the page. He gives him three minutes and Izuku attacks but Nighteye uses his Quirk to predict and evade Izuku's attempts. Nighteye is not too fazed by Izuku using Torino's movements in his efforts and taunts the boy who struggles to get the stamp but to no avail.

Eventually, Izuku runs out of time and Nighteye remarks that Mirio was a more suitable candidate for One For All. However, Nighteye is surprised when he notices Izuku didn't damage any of his collection of All Might merchandise, despite his leaping around and Nighteye's provocations. Impressed, he hires the young man into his agency.

The next day Nighteye takes his new intern along with Bubble Girl and Mirio on a patrol as part of their investigation on Shie Hassaikai. He shows them all a picture of their leader Kai Chisaki and then begins patrolling with Bubble Girl, having Mirio escort Izuku to show him the ropes.

Sir Nighteye and Bubble Girl stake out near Overhaul's hideout, noting that it had been nearly a week and a half since they began watching Overhaul's movements. Sir Nighteye notices that there are fewer people going in and out than usual. Suddenly, Bubble Girl gets a phone call from Mirio, who tells her that he had encountered Overhaul.

Meeting up with them, Mirio apologizes for not handling the situation properly but Nighteye tells him that it was his own fault for not using his Quirk to look at the boys and foreseeing the incident to prevent it from happening. Mirio then says that they had acquired new information; Kai had a daughter by the name of Eri, and shares with him what they learned about the girl. Izuku, on the other hand, expresses his desire to save her but Sir Nighteye tells him not to be arrogant. They can't rush to save anyone and need to go about it the correct way, without letting the villains know about their intention. The patrol then ends for the day.

A few days later, Nighteye holds a meeting in his offices with the Work-Studies gang and several heroes like Eraser Head, Gran Torino, Fat Gum, Ryukyu, and many more. He reveals that he called for this meeting to share information as well as to discuss how to proceed in dealing with the plots of a certain small organization named the Shie Hassaikai; this captures Eijiro, Izuku, Ochaco and Tsuyu's attention.

Nighteye explains that Kai, the Leader of the Shie Hassaikai, has a Quirk called Overhaul. Kai's Overhaul Quirk gives him the ability to disassemble or restore objects, an ability that can break down something and then reassemble it. Sir Nighteye states that Kai's Quirk and the Quirk-Destroying Bullets are related.

Nighteye explains that Kai's daughter had no birth certificate and details about her are unknown; Izuku and Mirio encountered her on their patrol and she had a number of bandages on her. Gran Torino comments that since they live in a superhuman society, anyone could do anything if they put their mind to it. Eijiro asks what Gran Torino meant by this and Sir Nighteye explains that Kai is probably using his daughter to make those Quirk destroying bullets, which disgusts Eijiro, Ochaco, and Tsuyu.

However, Sir Nighteye deems the bullet used on Tamaki to be a prototype since it did not destroy Tamaki's Quirk and they don't have evidence that Kai is selling the bullets. Fat Gum is disgusted with Kai's crimes and demands that they hunt them down. One of the Heroes, Rock Lock, states that this problem would have been solved already if Izuku and Mirio took Kai's daughter into custody. Sir Nighteye states that he takes full responsibility for that and does not want him attacking them since it is his fault that he prevented them from rescuing the girl. A frustrated Izuku and Mirio lament their failure in attempting to rescue Eri and condemn themselves for thinking that they could become the best Heroes.

Izuku and Mirio stand up and declare that this time they will definitely rescue Eri. Sir Nighteye states that their objective will be to save Kai's daughter.

Seeing that the meeting is currently going nowhere, Shota Aizawa asks Sir Nighteye about his Quirk ability; since he can see into the future, he can see the outcome of their rescue operation. Nighteye reveals that he is unable to. Nighteye explains that his Quirk has a twenty-four-hour recharge period before he can use it again, meaning that for one hour every day he can see one person's future.

Similar to a series of flashbacks frame-by-frame, Nighteye can see the film of an individual's life; however, the future he sees is from the perspective of the person and all he can foresee is that person's actions and immediate surroundings. Shota is somewhat confused since Nighteye's Foresight will give them more than enough information and asks why he cannot. Nighteye gives an example; if he sees a person's death awaiting them, there is nothing he can do about it.

Much later, he talks with Torino and asks him if the reason he directed Izuku towards him was to mediate his relationship with All Might. Gran Torino denies it and saying he thought it could give them a chance, though it was just a thought that crossed his mind and asks what he thinks. Sir says Deku is similar to All Might including the madness in their core that he could never understand.

Two days later, all the Heroes report to Sir Nighteye's office where he reveals that Eri is in the Shie Hassaikai Compound. Sir Nighteye shows a toy intended for little girls and assumed that the person buying the toy was a member of the Shie Hassaikai. Fat Gum remarks that the person buying the doll was just some enthusiast.

Sir Nighteye corrects him by telling him that he used his Foresight Quirk on the suspect to see his future and discovered that he is Eri's cater, and thanks to that he knows how to get to her. The sidekicks of Sir Nighteye's Office state that they have a time frame of when they will be home while Gran Torino remarks that they have the warrant as well.

These good news makes Mirio recover part of his energetic personality, and tries to encourage Deku, which pleases him and the classmates of both. Seeing his apprentice, Sir Nighteye understands how much Mirio has suffered in recent days for having decided not to rescue Eri when he had the chance, prioritizing obtaining the best possible result of that entire operation, but he is sure that his past decisions do not will have been in vain once they have rescued the girl.

The next morning at 8:00 AM, in front of the Police Station, Heroes partaking in the rescue operation have been given detailed information about the Shie Hassaikai's members and the underground facility. Sir Nighteye later talks with a man who tells him that Gran Torino is unable to attend the operation, due to something coming up involving the League of Villains.

At 8:30 AM, the Heroes and Police Force are outside the Shie Hassaikai's Headquarters and commence the operation. They are attacked by surprise by Rikiya, who is stopped by Ryukyu, who assures her agency can deal with the villain and asks the other groups to rush to the hideout. Sir Nighteye and Fat Gum's groups charged into the Shie Hassaikai's main building.

Rock Lock has a bad feeling since something is wrong due to them while Shota assumes that since Kai has not shown himself, he must be in the basement evacuating everything to hide which includes Eri. The Heroes reach a shrine with a vase on it; Sir Nighteye removes the vase and pushes on a wooden panel, which open a pathway to the underground labyrinth. They are attacked by some Yakuza thugs, who are quickly subdued by Bubble Girl and Centipeder, allowing Nighteye and the others to proceed. They quickly run into a blockade that impedes their path through the underground maze. As the wall does not appear in the planes, Sir Nighteye deduces that it has been created by Overhaul's Quirk.

Izuku and Eijiro break down the wall, but they're impeded yet again. Powered by a Improvement Drug, Joi Irinaka, the general manager of Shie Hassaikai known as Mimic, uses his Quirk to contorting and twisting the walls to make the pathway difficult to navigate. Mirio goes on ahead by himself while the others fall through a hole created by Mimic to another room. They come face to face with Toya Setsuno, Yu Hojo, and Soramitsu Tabe of the Eight Bullets. Tamaki decides to deal with them alone while the rest continue the persecution.

Sir Nighteye's group is having trouble traversing the basement due to Mimic's interference. In one of his attacks he manage to separate Fat Gum and Eijiro from the group, but the rest continues. Later, Mimic uses his Quirk to close everything in an attempt to crush Sir Nighteye's group but Rock Lock uses his Ultimate Move Deadbolt to keep a stable area around them. After failing several attempts to crush them, Mimic opens up the passageway before warping the path again with sudden attacks from the roof. He successfully splits up the hero squad into three sections: Eraser Head and Deku, Rock Lock alone, and Sir Nighteye with the remaining Police Force.

While Izuku, Shota and Rock Lock deal with Himiko Toga, Nighteye has to face Twice, who has used his Quirk to create a clone of Kendo Rappa, one of the Eight Bullets. Twice orders the Kendo Rappa clone to attack, who rushes Nighteye and the Police with a barrage of bullet punches. Nighteye easily predicts the clone's movements, evades them, and swiftly counters the clone with a tripping kick. The clone grazes him, tearing his clothes. In response, Sir Nighteye sends the clone flying into the wall using several Hyper-Density Seals, which surprises Twice along with the Police Force members.

Sir Nighteye explains that when it comes to close combat and prediction he is quite fast, but what he did not predict was the League of Villains teaming up with the Shie Hassaikai. As the Rappa clone dissolves, Twice blames the yakuza for being useless. While he rants, Sir Nighteye strikes Twice with the seals and rips off a part of his mask, forcing him to retreat behind a wall created by Mimic. For his part, Himiko Toga is also forced to retreat after facing Izuku and Shota.

Without anyone being able to predict it, Himiko and Twice decide to betray the Shie Hassaikai, causing the heroes to found Mimic's position. Izuku destroys his hiding place, Shota cancels his Quirk, and Nighteye knocks him out with one of his Hyper-Density Seals. Nighteye's team successfully works with the police to restrain Joi Irinaka.

The chief demands Irinaka return the walls to their normal form, but he can't do it because the enhancer drug has stopped working. Sir Nighteye asked Joi about the location of the other members of the League, but Joi did not know and promised that when he found them, he would kill them. Sir Nighteye understands that this means there aren't any more Leaguers other than Twice and Himiko. Despite his injuries, Rock Lock pushes everyone to move along and finish the mission. Nighteye, Izuku and Shota leaves Mimic under Rock Lock and the police's watch.

The trio arrives in time to help Mirio in his confrontation against Overhaul. While Izuku and Shota face Overhaul, Sir Nighyeye approaches his wounded pupil. He is amazed that Mirio Togata has been fighting all by himself against the boss of the Shie Hassaikai and his closest aides by himself and not only managed to defeat the aides (Chronostasis, Shin Nemoto and Deidoro Sakaki) as well as injuring Chisaki but on top of all that, he 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.

But the fight is not over yet. Overhaul managed to heal Hari during their fight with Mirio, and at his command, Chronno uses his Quirk and cuts Eraser Head's arm. The hero was trying to save his student, but now his body moves slower as a result of Hari Kurono's power. Shota is forced to blink due to the cut from Chronostasis, which allows Chisaki to use his Quirk again. Chisaki belittles the Heroes' efforts as he constructs massive spikes from the ground to attack the heroes.

Irate, Overhaul refuses to allow his plans to end here, and uses his Quirk to fuse with Nemoto to grant Overhaul a new, stronger, and more monstrous form. Armed with four new arms, Overhaul demands the heroes return Eri. Deku notices that Chrono and Aizawa are gone, while Lemillion and Eri are being cared for by Nighteye. Overhaul taunts Mirio, announcing to his allies that his Quirk has been erased for good in his desire to protect Eri at all costs. Deku and Sir Nighteye are shocked to learn that Mirio's Quirk has been lost forever.

Overhaul rushes at Sir Nighteye, who's protecting Mirio and Eri, but Deku faces him to protect them. After a short struggle, Sir Nighteye intervenes, throwing his Hyper-Density Seals at Overhaul. Nighteye steps up to fight Overhaul and orders Izuku to take Lemillion and Eri away while while he faces the villain.

Sir Nighteye demands to know about Shota's whereabouts, to which Kai reveals that he took an interest in Shota's Erasure Quirk and his subordinates are escorting him to the VIP room, causing Nighteye to wonder if Chisaki is afraid of having his own Quirk destroyed. Then he claims Overhaul must have been very afraid of Lemillion if he was willing to spend one of his precious perfected bullets on him.

Kai attacks but Sir Nighteye is able to evade his attacks. Sir Nighteye thinks back to the lessons he taught Mirio and is happy as well as proud that he took his teachings to heart. In return, Sir Nighteye vows to keep him and the person whom he wants to protect out of harm's way. Nighteye continues to put a fight while using his Quirk to predict Kai's movements.

During the battle between him and Chisaki, Sir Nighteye activated his Foresight Quirk. Whatever future he sees cannot be changed; this is the absolute rule as he himself has tried to change the future to no avail. Ever since he saw All Might's death, Sir Nighteye decided to never read people's futures and would only looks seconds and minutes into the future from then on.

Sir Nighteye did everything in his power during the battle to deny Chisaki the future that was in his grasp and even though his actions were drawing out the inevitable, this was the best course of action he could take. Unfortunately, Sir Nighteye reached his limit and was impaled by Chisaki's reconstructed rock spikes. Having touched Chisaki, Sir Nighteye saw into his future. Sir Nighteye held onto a sliver of hope that Eri would be saved, the boys safe and Chisaki defeated as well as imprisoned. However, a good future doesn't exist and he can only see himself and Deku dying at the hands of Overhaul, who manages to escape with Eri.

After defeating Sir Nighteye, Overhaul pursues Eri, forcing Deku to leave her to Mirio and faces the villain. Izuku presents a tough fight against Overhaul, but not even with the One For All: Full Cowl to 20% he can defeat him, and ends up being seriously injured by the villain. Wanting to end the fight, a mouth appears on Chisaki generates a mouth on one of his hands and uses Nemoto's Quirk to psychologically attack Eri and force her to return to him.

His plans works, and the girl returns, but Izuku refuses to give up. Despite the future showing Nighteye that Overhaul wins, in spite of Eri's desire to sacrifice herself and not to be saved, Deku refuses to let anyone die and claims he'll save Eri no matter what. Suddenly, Ryukyu's team crashes through the roof with Rikiya Katsukame in tow.

It turns out that Ryukyu's team was tricked by Himiko and Twice to get to where Overhaul was, sending a clone of Mr. Compress to snatch Eri. Izuku tells Ochaco that he is leaving Sir Nighteye to her as he goes to rescue Eri. The struggle with Overhaul ends with Eri activating her Quirk Rewind and undoing the fusion of Overhaul with Nemoto, then fleeing to Deku's arms. She and Deku accidentally surfacing when Izuku unconsciously used the One For All at 100 % to dodge an Overhaul attack.

Obsessed with seeing his plan through until the end, Overhaul heals his injuries and merges his own body with Rikiya Katsukame's. Ryuko Tatsuma prepares for battle, but a grievously injured Sir Nighteye, who is being tended to by Tsuyu Asui, eases her worries when he claims that his Foresight has dictated that Overhaul will go after Izuku and kill him to get Eri back.

Despite her exhaustion as well as the horrible revelation, Ochaco Uraraka still wishes to try and help Izuku. Ryukyu also refuses to let Chisaki go after Izuku. However, Sir Nighteye convinces her that her group cannot hope to defeat the newly fused and empowered Chisaki. Ochaco refuses to stand by and do nothing. Despite his belief, he thinks about Izuku's declaration of changing the future. Sir Nighteye asks Tsuyu to tend to Mirio and for Ochaco and Ryukyu to take him up to the surface.

Once at the surface, he and Ochaco witness Izuku defeating Overhaul, in contrast to his vision. Ochaco questions Sir Nighteye's vision about Izuku dying. Sir Nighteye replies that it is the unchangeable future he saw but is surprised that events are playing out differently. Sir Nighteye smiles when Deku twists fate by defeating Overhaul, proving that the predictions of his Foresight can be changed.

After the battle is over, the Police Force and Heroes search the Shie Hassaikai's hideout, capturing the defeated Shie Hassaikai's members, take over Overhaul's experiments as well as carrying the injured Heroes to the ambulances.

Izuku rushes to Sir Nighteye and tells him that All Might said that he is alive and is ashamed to look at him in the eye. Izuku asks Sir Nighteye to hang on long enough for him to see All Might again. Sir Nighteye closes his eyes, replying to Izuku that many Heroes were injured to save one girl and, although he finds the situation difficult to be pleased with, thanks Izuku. At 9:15 AM, the Pro Heroes rescue operation to save Eri has been completed.

Sir Nighteye is sent to the Hospital but his wounds are too serious. Accompanied by Shota, Izuku goes to see Nighteye, meeting All Might and Recovery Girl who are with Bubble Girl and Centipeder. Bubble Girl tries to explain that she invited All Might for a reason but breaks down in tears before explaining as Centipeder consoles her. The surgeon explains that there is nothing they can do for Sir Nighteye, and Recovery Girl explains that her Quirk will not help either, much to Izuku's shock. The surgeon apologizes and states that Sir Nighteye will not live long enough to see tomorrow. Everyone goes to Sir Nighteye's room.

Sir Nighteye opens his eyes and sees All Might, finding it humorous that his mentor finally visits him one last time on his deathbed. All Might does not know how to reply and is full of remorse for not visiting him before. Izuku begs Sir Nighteye to live as Sir Nighteye claims that he does not blame All Might and only wanted him to be happy and is fine with All Might fighting against fate. All Might begs his former sidekick to fight against fate as well and wants to make amends for everything he has done.

Nighteye tells Izuku that he was wrong about him as he was able to twist fate. Nighteye doesn't dwell on his fate as he states Izuku has shown him a way to change the future when he previously failed to find a way to stop All Might's impending death. Sir Nighteye hypothesizes that everyone's wishes for a better future changed the outcome. All Might and Izuku grieve as Sir Nighteye is satisfied with his changed views of the future never being certain. However, there is one thing he regrets.

At that moment, Mirio run into the room and frantically begs his mentor not to die, as he still had much to learn from him. Sir Nighteye is apologetic for making Mirio experience much hardship but a weeping Mirio still begs Sir Nighteye to live because thanks to him he is the man he is now today. He 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 for once thinking of Mirio as nothing more than a vessel.

As Mirio weeps, Sir Nighteye raises his arm, touches Mirio's face and uses his Foresight Quirk for the last time to see Mirio's future. Nighteye comforts his pupil by telling him he will become an outstanding Hero, saying that his future is the only one that should not be changed. Sir Nighteye closes his eyes and smiles, telling everyone to keep smiling as society needs smiles and laughs to bring about a brighter future as All Might, Izuku, and Mirio weep.

Sir Nighteye passes away peacefully, with a smile on his face.

Remedial Course Arc
The work-study gang, accompanied by All Might, Shota Aizawa, Fat Gum, Ryukyu, and Nighteye's sidekicks, attend his funeral. Centipeder takes over Nighteye's Office, and he and Bubble Girl tell Mirio that he is welcome back anytime.