Nemuri was once a student at U.A. High School. At one point, she participated in the Hero Work-Studies at the Purple Revolution Agency being a trainee to His Purple Highness. During her third year, she began a friendship with the second year students Shota Aizawa, Hizashi Yamada and Oboro Shirakumo.
It all started on her way home from her Work-Study, she went to school to do some paperwork, stopped by on one the rooftops because it's one of her favorite spots. Once there, she found the three students eating their lunch there, and she warned them that the roofs are off-limits to students, but when it seemed that she was going to admonish them, she actually praised them for their rebellious attitude.
The boys were surprised to find her there, and Shota asked her about her hero costume, because it was too revealing, to the point she was almost naked. Although she knew her costume would be scorned and questioned, she told it's bold yet practical, to support both her Quirk and moves, and she didn't feel ashamed for wearing it. in fact, she was convinced that she would change the opinion of the Japanese in one year's time. Little does she know that soon, her costume would lead the government to put forward the "Hero Costume Skin Exposure Limitation Act."
Nemuri stopped talking about herself and spots the cat Shota was holding. Oboro explained that him and Shota were taking care of it when Nemuri said they really shouldn't have animals at school and swipes the cat away. Nemuri asked for the name of the cat, and not having it, decided to name it sushi after asking the boys what their favorite food were. She said she would have the kitten temporally until the trio found him a proper home, because she planned to leave home after graduation, so she won't be in any place to care for it. Then, she left the roof with the kitten, and Oboro stated she will end up keeping him.
The next day, Nemuri returned to the roof, meeting Shota and Oboro eating. After informing them that Sushi was doing well, she told them she had a proposition. Knowing that both Shota and Oboro were not yet assigned to an Hero Office to participate in the Work Studies, she informed them that her boss was willing to take on anyone.
Shota and Oboro accepted her offer and began their work studies with His Purple Highness, forming the Team Purple Revolution. A day on the field, a robbery was happening when Oboro and Nemuri entered the scene, forcing the Robber Villain to use his Smoke Quirk to escape. Oboro wanted to chase after him, but Nemuri stopped him, arguing that the smoke could be toxic and they should prioritize evacuating the citizens. Oboro yelled at Shota to catch the thief, but he managed to escape thanks to his Quirk.
The three students apologized to His Purple Highness for failing, but the hero praised them and gave the trio his feedback about what they should improve, especially Shota. Days later, they met the Robber Villain again, and this time they managed to capture him.
For a while, the students of class 2-A returned to the U.A. to show their progress and participate in a Two on Two Battle Training together with class 2-B. After the exercise, Nemuri met Shota, Oboro and Hizashi hanging out on the rooftop again, talking about their plans to open an agency together once they graduate. Nemuri admires the youths bantering and Oboro asked her to join them. Nemuri declined the offer, since she already had her own plans once she goes independent, with young people waiting for her. Then, Oboro stated he wouldn't mind waiting on her and would later give her a pair of googles.
After this brief period in the U.A., Nemuri, Oboro and Shota returned to the Purple Revolution Agency to continue with the Work-Studies. The Purple Revolution team patrolled the streets of Tasomiya Ward in two groups: one formed by Shota and Oboro, and the other by Nemuri and Mr. Purple.
During the patrol, a villain known as Garvey made an appearance, causing great destruction in the city. Nemuri and His Purple Highness witnessed how the hero team Buster Union tried to stop him, but were easily defeated by the villain. Midnight used the Hero Network to obtain information and reported to her boss about the villain’s criminal record and his Stock Quirk, showing concern because the data did not indicate that Garvey had as much destruction capacity as he was demonstrating at the time. Mr. Purple ordered Midnight to call for backup and help evacuate the area while he faces the villain to buy time.
His Purple Highness, Shota and Oboro faced the villain to protect a group of schoolkids. In the end, Shota was able to defeat the villain alone, but the victory came at a price. Mr. Purple had to be hospitalized for the injuries that he suffered and Nemuri and Shota discovered that their friend, Oboro, had died during the incident while protecting the children.
Nemuri could not help but cry over his death, while Shota and Hizashi were mourning in the place where Oboro had died, not caring about the heavy rain at that moment. After his body was removed from the area, Nemuri told Shota and Hizashi to cover themselves if they didn't want to end up getting sick. Hizashi thanked her for her concern, but told they were fine and continued in the area for hours.
Months before she started working as a teacher at the U.A. High School, Nemuri was involved in a series of events caused by a criminal organization known as the Villain Factory, a criminal organization responsible for trafficking Trigger, an drug that boost Quirk but turns its consumers in violent Instant Villains.
One day, one of the Factory's agents, Kuin Hachisuka, uses her Queen Bee Quirk to inject Trigger into innocent civilians, causing an Instant Villain Outbreak. Detective Naomasa immediately call for backup. Nemuri and other heroes appear immediately and quickly they suppress the outbreak. 
Due to the villain outbreak, where several innocent people were turned into Instant Villains, detective Tanuma organized a meeting in the Police Station with multiple Pro Heroes who helped to stop the outbreak, to discuss the situation the best action to take, Midnight being one of them.
After the meeting, she Midnight approaches Eraser Head asking about where Present Mic is because he was supposed to be at the meeting. He explains that Present Mic had a cold and could not make it. Midnight scolds Eraser Head on his attire. In response, he teases her saying she should become a teacher or something if she likes counseling others so much.
Much to his chagrin, she tells him that is exactly what she planned on doing. Continuing on she reveals that principal Nezu personally requested her and that she will start next year at U.A. She details in constant innuendos her beliefs on teaching. Aizawa disagrees with her methods and as she began a rebuttal. Then, a villain called Suicide Bomb attacks the police station, threatening to kill everybody. Endeavor still fuming over what has just transpired decides he is going to incinerate the villain.
However, the villain's Quirk will most likely have a volatile reaction to Endeavor's Hellflame. Midnight calls Eraser Head to help resolve the problem before it gets explosive. Eraser Head wraps up the villain and threw him into Midnight, who calms him and passes him out with her Quirk. The Midnight Boys come bursting in asking if Midnight is unharmed. She tells them she is fine and orders them to hand the villain over to police custody.
Midnight returns to her conversation with Shota Aizawa telling him he should also become a teacher because a wide variety of opinions and teaching styles is required to educate the youth. He asks if she is joking about him becoming a teacher because that would be the epitome of irrational. Despite his words, Midnight submits a recommendation for him to the principal, informing Shota the next day about her decision.
Weeks later, she is invited by Makoto to participate along with other heroes in a musical event organized by the Marukane. Midnight accepts the invitation. In the greenroom, he meets Preesent Mic, and ask him about Eraser Head whereabouts. Present Mic replies that Shota told him he would come to the show if he can. Midnight assumes he won't come. As she predicted, Shota cannot attend because he has to face a villain that generates a blackout in the area. Fortunately, Makoto's team fixes the generators and the festival continues, ending up being a success.
Several days later, taking advantage of her day off, Nemuri decides to go to Narufest to enjoy the event. To her surprise, she meets Shota Aizawa present at such a bustling event. He responds that he is on duty and stopping by for lunch. Shota asks her why she would be attending on her day off and Nemuri responds that it is because she enjoys such events. Soon, detective Naomasa arrives and Nemuri introduce herself to him, who does not recognize her at first because she did not wear her characteristic hero costume.
Although Naomasa is there to talk to Shota, he decides to include Nemuri in the conversation about Teruo Unagisawa, the villain Shota had captured the night of the festival in Marukame. He tells them that Teruo had been arrested twice, however between his first and second arrests his body was drastically bioengineered, the police had found several other villains with similar alterations with high concentrations of Trigger present in them. The police decided to name this new threat Next-Level Villains
The police suspect that there is an organization behind the distribution of Trigger that is also responsible for the Next-level Villains Shota says that the organization must have some greater goal in mind. Naomasa agrees and ask Shota for his help.  At first, Eraser Head is not very sure of participating, because acting as a detective is not his thing, but Nemuri Kayama convinces him to help the police.
Nemuri also helps Naomasa occasionally in the investigation. Police found out that there is a new version of the Trigger, that only needs to be ingested to work, and there is groups who are distributing Trigger at mixers, causing several incidents. Originally Naomasa Tsukauchi wanted Shota Aizawa to carry out the mission of arresting the suspects of this practice, but he refused, saying he did not take on assignments that required high-energy and social activity, and instead he recommended Midnight to tackle the task.
It turns out that for the mission to which she was entrusted, she coincides with Koichi and Kazuho, the Vigilantes The Crawler and Pop☆Step respectively. The Mad Hatters invited Koichi to participate in a mixer and Koichi accepted, much to Kazuho's disgust. Later, Midnight has run into Kazuho as the latter is tailing Koichi on his way to the mixer, recognizing her as Pop☆Step who she had met at the first Narufest. Then, Midnight sees the main suspect of the incidents, Rin Yaritezawa, accompanied by her friend Hina, and outlines plan to also participate in the Mixer together with Kazuho. Midnight telephones the Midnight Boys to distract the other two girls who would join the Mixer, getting them to cancel their participation, and so she and Kazuho take their places as "substitutes".
During the Mixer, a series of events make Rin fall in love with Koichi, which unleashes Hina’s jealousy and decides to take revenge on Koichi, mixing in his drink a few drops of Trigger. However, due to a number of circumstances, the drinks that she offers to Koichi end up being drinks for other customers of the bar, resulting in them starting to lose control over their quirks. Midnight uses his Somnambulist Quirk to put everyone to sleep, and arresting Hina in the process, who turns out to be responsible for the previous incidents at mixers, by drugging guys to keep them from talking to Rin. The next morning, Midnight is congratulated by Naomasa for the work done, and Eraser Head could not agree more.
Months later, on the occasion of the return of Captain Celebrity to the USA. Makoto decides to organize a farewell show at the Tokyo Sky Egg, inviting several of Japan's most famous heroes to the event, including Midnight.
In the middle of the event, the Tokyo Sky Egg is attacked by creatures known as Bombers, commanded by the villain Number 6, a member of the Villain Factory. The main objective of Number 6 is to kill Captain Celebrity, and to achieve it he orders the bombers to damage the structure, leaving the Captain celebrity at his mercy by having to use his strength to prevent the building from collapsing, leaving him completely defenseless to the Bombers attack.
The heroes who were participating in the event decide to organize in two groups: one group goes outside and confronts the enemies, while another group helps the show staff to look after the spectators who had come to enjoy the show without telling them the truth about what is happening, to avoid panic. Midnight is part of this last group. Thanks to the intervention of the heroes, and the timely arrival of All Might, they manage to avoid a tragedy.
Weeks after the Sky Egg Bombing, Midnight phones Shota Aizawa to talk about her experience as a teacher at the U.A. and tell him that Hizashi Yamada will start teaching next semester, and the only one missing is him. Shota replies that he has not agreed to being a teacher, but Midnight replies that according to Present Mic he only needs a good shove to take action. Also she tells Shota that he has been shying away from what really matters. Pissed off at Midnight's words, Shota ends the conversation. Although after re-thinking, he accepts the offer to be a teacher.
Prior to the start of the U.A. Sports Festival, Midnight introduces herself as the chief referee. She warns everyone to play fair while all the students question if her revealing attire is appropriate for a high school event. She demands all of them be silent before she summons Katsuki Bakugo to the podium to lead the pledge. After he finishes, Midnight explains that the preliminary event will be an Obstacle Race around the stadium.
Following the Obstacle Race, Midnight reveals the results and how it translates to the next event: the Cavalry Battle. Midnight explains the rules of the Cavalry battle and its point system, revealing that Izuku Midoriya will be worth the most points since he came in first. While explaining the rules, she is interrupted by Mina Ashido and Ochaco Uraraka. This angers Midnight, prompting her to yell at the girls to allow her to finish explaining the rules. She warns the teams not to knock each other over and allows them to choose their teammates.
When the time for the event comes, Midnight leads the countdown. She allows Minoru Mineta and Tsuyu Asui to hide on Mezo Shoji's back, and when Katsuki uses his explosions to fly away from his team, Midnight allows it because he never touches the ground. Midnight announces when the Cavalry Battle ends and allows Present Mic to announce the placements.
After the lunch break, Midnight announces that those who qualified for the finals will draw lots to determine the pairings for a one on one fighting tournament. She also informs everyone who did not make it may participate in recreational events prior to the finals. She is interrupted when Mashirao Ojiro asks to resign from the finals because he felt he did not play a big enough role in the second round. Nirengeki Shoda feels the same way, causing Midnight to get "turned on" by their naivety. She allows them to resign and says that two members from Team Kendo must fill in their spots. Team Kendo agrees they did not do much in the Cavalry Battle either and allows Tetsutetsu Tetsutetsu and Ibara Shiozaki to take the two open slots.
Midnight oversees each of the matches during the fighting tournament. She is half frozen when Shoto Todoroki creates a giant ice ridge to freeze Hanta Sero. Even so, she does her job and asks if Hanta can move, to which he replies obviously cannot. When Eijiro Kirishima and Tetsutetsu fight to a draw, Midnight announces that their match will be settled via arm wrestling once they recover. Midnight is forced to stop the match between Katsuki and Ochaco when Ochaco's body reaches its limit and she falls unconscious.
During the match between Izuku and Shoto, Cementoss contacts Midnight and asks her if they should stop the match because of Izuku's injuries. During the climax of their bout, Midnight tries to use her Quirk to knock them out, but is knocked off her stand by the explosion at the matches end. Once the smoke settles, Midnight sees Izuku has been knocked out of bounds and announces Shoto as the winner.
When the final match between Katsuki and Shoto concludes, Katsuki angrily confronts Shoto although he has been incapacitated and knocked out of bounds. In order to stop Katsuki's rage, Midnight uses her Quirk to knock him out and restrain him. Midnight introduces All Might to close out the Sports Festival but ends up interrupting his entrance by accident, though she apologizes for it.
Following the U.A. Sports Festival, Class 1-A is among those selected by Pro-Heroes to be drafted for internships. Shota requests Midnight come to their homeroom class to help them choose hero names. She has everyone come up with their own names and present them in front of the class, approving or disproving them as they go one by one.
Midnight is featured on a daytime talk show where she talks about the necessity of her need to dress sexily while doing hero work. She explains that she uses a thin costume because, like many others, her Quirk requires exposed skin to work. While talking, she is repeatedly interrupted by Mt. Lady, who is also being interviewed. Eventually, the two come to blows over their ages, much to Minoru's viewing pleasure.
Later on, Midnight meets with her fellow faculty members prior to Class 1-A's final exams. They discuss changing the finals format from facing robotic villains to fighting against the teachers. Midnight comments that this may be too difficult for the students, but they all ultimately agree on the change. Cementoss joins the other teachers at the Final Exams practical portion test site to meet Class 1-A.Midnight is placed against Minoru and Hanta in the ninth match.
At the start of their bout, Midnight uses her Quirk to put Hanta to sleep, and sits at the escape gate with a sleeping Hanta on her lap, much to Minoru's jealousy. Midnight decides to pursue her fleeing student because her sadistic side is aching and whips him as he runs away from her. She criticizes him for running away, but Minoru reveals that his cowardice was just a ploy to fan Midnight's sadistic side and lead her away from the escape gate.
Minoru reveals as a fan of hers, he knows her quirk and has a plan. He comes out of hiding with Hanta's tape wrapped around his mouth to avoid breathing in Midnight's fragrance. She claims he cannot fight her while suffocating, but he uses his Grape Rush technique to stick his spheres to her whip and immobilize her. Minoru runs past her and Midnight admits that Minoru was very impressive. Minoru grabs Hanta and drags him outside the gate to pass the exam.
Midnight, Ectoplasm, and Cementoss join Shota Aizawa in his homeroom class to announce that Class 1-A needs to develop super moves. They move to Gym Gamma where the teachers announce that the students will need special moves for the upcoming Provisional Hero License Exam. Midnight uses Kamui Woods' Lacquered Chains Prison as an example of a super move. Four days later, Fumikage Tokoyami develops his own super move. Midnight advises him to change the name because super moves should be easy to say. 
Midnight expresses her concern that Izuku might not make it in time for Class 1A's performance for the School Festival when Toshinori tells her that Izuku received permission to leave school grounds to buy last minute supplies.