Mirai Sasaki/Synopsis

History
Sir Nighteye 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, Nighteye was so persistent that All Might eventually let him be his sidekick. Sir Nighteye took on a supporting role, being the brains of his operations, while All Might fought on the front lines. And 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 knew about the secret of Toshinori Yagi and the One For All nature.

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 defeating him, All Might received critical injuries: His respiratory system was completely damaged and that set a limit on the usage of his Quirk. Sir Nighteye begs All Might to retire due to this, as he doesn't want All Might to push himself to the brink of death. All Might refuses to since the world is waiting for his return and he must answer their pleas. Sir Nighteye begs All Might to retire and let his legend live on for the sake of the peace he desires. Nezu agrees with Sir Nighteye and tells All Might that he can look for a One For All successor at U.A., to which Sir Nighteye finds Nezu's idea to be the best solution.

All Might asks who the Symbol of Peace will be until he finds that successor. Even though All For One is gone, another villain like him will eventually appear. Sir Nighteye states that he will stop supporting him if he plans on continuing as a Hero. All Might notices that Nighteye was "looking" at him with his Quirk. Sir Nighteye tries getting All Might to agree, saying that even if he retires, the next Number One Hero will eventually rise and although that "short while" will be difficult they may be able to ward the villains off.

However, All Might still refuses since that short while will be a period of fear. All Might notes that Nighteye's Foresight is never wrong, to which Sir Nighteye fears as he has seen what will happen to All Might at this rate. Sir Nighteye tells 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 head towards a gruesome death.

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

Years later, All Might meet Izuku and decides he is 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 on the phone regarding Izuku; Toshinori argued that Izuku was intent on saving others while Nighteye argued that intentions were not good enough since there are more capable candidates with the same intentions as Izuku. However, Toshinori stated that Izuku is Quirkless which made him a befitting candidate over the others. Toshinori says 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.

Internship 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; Chisaki had a daughter by the name of Eri. The patrol then ends for the day.

Izuku goes and confronts All Might who tells him about his past with Sir Nighteye. It is revealed that All Might initially would not take Nighteye on since he had a rule to not have any sidekicks. However, Nighteye's persistence eventually convinced All Might to break down and accept him into the position. They worked as a team for five years, then after All Might's battle with All For One and the resulting crippling injury Nighteye insisted that All Might retire. However, the number one hero refused, saying that his duty as Symbol of Peace meant that he could not leave the people for fear of another villain as bad as All For One emerging and terrorizing them. Nighteye tells All Might he used his foresight on him and saw that he would clash with a villain and meet a gruesome demise. The two had a falling out at this and broke up as a team.

A few days later, Nighteye holds a meeting regarding Shie Hassaikai in his offices. 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 Eight Precepts of Death; this captures Eijiro, Izuku, Ochaco and Tsuyu's attention. Nighteye explains that Chisaki, the Leader of the Eight Precepts of Death, has a Quirk called Overhaul. Chisaki'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 Chisaki's Quirk and the Quirk-destroying bullet are related.

Nighteye explains that Chisaki'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 Chisaki 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 Chisaki is selling the bullets. Fatgum is disgusted with Chisaki's crimes and demands that they hunt them down. One of the Heroes states that this problem would have been solved already if Izuku and Mirio took Chisaki'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 Chisaki'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 says it was just a thought that crossed his mind and asks what he thinks. Sir says they're both similar 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 Eight Precepts of Death Headquarters. Sir Nighteye shows a toy intended for little girls and assumed that the person buying the toy was a member of the Eight Precepts of Death. Fatgum remarks that the person buying the doll was just some enthusiast. However, at the toy store, Nighteye spotted Eri's caretaker buying a toy. Sir Nighteye touches the caretaker's shoulder and uses his Foresight Quirk ability; Nighteye sees the man's future involving Eri. 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.

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.

Sir Nighteye and Fatgum's groups charged into the Eight Precepts of Death's Headquarters. Lock Down has a bad feeling since something is wrong due to them while Shota assumes that since Chisaki 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 causes a door to open.

During the raid, Sir Nighteye battles a clone of Kendo Rappa and is able to hold his own against the latter.

Later on, he confronts Kai Chisaki and asks about Shota's whereabouts, to which Chisaki reveals that he took an interest in Shota's Erasure Quirk and his subordinates are escorting him to the meeting room, causing Sir Nighteye to wonder if Chisaki is afraid of having his own Quirk destroyed. Sir Nighteye assumes that Chisaki has completed his Quirk Destroying bullets and used one on Mirio. Sir Nighteye mocks Chisaki for being afraid of Mirio's power as Chisaki creates more arms for himself to attack by using his Quirk. Chisaki 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 Chisaki's movements and is able to last a good while but he is soon impaled through the chest by Chisaki.

Grievously injured by the villain's attack, Sir Nighteye tells the arriving Ochaco Uraraka and Tsuyu Asui that he has foreseen that Izuku will be killed and their attempts to save him won't matter.

Despite his belief, he has Ochaco use her quirk to take him up to the surface. He thinks about Izuku's declaration of changing the future. Once at the surface, he witnesses Izuku defeating Chisaki, in contrast to his vision. After the battle is over, Nighteye compliments Izuku on his actions and states he has gone beyond expectation. Izuku tells him to take it easy as he is taken to the hospital.

At the hospital, doctors tell his visitors that he won't last. All Might who was called by Bubble Girl walks into the room, as Nighteye remarks on how it fitting it is for his mentor to visit him one last time on his deathbed. All Might laments on not visiting him and begs his old friend to pull through so he could make amends.

Nighteye tells Izuku that he was wrong about him as he was able to change the future after Izuku begs him not to die. 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. Mirio ran into the room and frantically begged his mentor not to leave, as he still had much to learn from him. Nighteye then comforted Mirio by using his Quirk one final time to tell him he'd be a great hero in the future. Nighteye gives his pupil his final words before succumbing to his wounds and passing on.