When he was three years old, he was in bed and woke up since he had to go to the bathroom. When he rubbed his eye, his Quirk manifested for the first time. This made his hand harden and caused Eijiro to cut himself, which left a scar on his right eye.
During Junior High, Eijiro and Mina attended the same school and although they were not close, Mina's outgoing nature and strong personality made Eijiro self conscious of his own appearance and personality. This was worsened during an incident where Eijiro found himself unable to stand up against a figure who was threatening fellow students of his school.
Afterwards, Eijiro accidentally came across an old holographic projector depicting an interview with Crimson Riot. The words of Eijiro's idol, which had inspired him once, again inspired him further to keep following his dream of going to his dream school, training himself in order to pass the the U.A. Entrance Exam.
The morning before classes started at U.A. Eijiro met up with Mina again. Sporting a new hairstyle, Eijiro claimed to have left his insecurities behind, but Mina playfully told Eijiro to not pressure himself too much and let the change just happen naturally once Eijiro became truly confident about himself.
Eijiro Kirishima is first introduced when Izuku Midoriya arrives on the first day of class. He is sitting at his desk, alongside the other Class 1-A students. Shortly afterwards, the class makes their way to the P.E grounds, where Shota Aizawa informs them they must undertake a Quirk apprehension test. Eijiro is shocked by his teacher's announcement that student who places in last will be expelled.
The next day at school, All Might teaches Class 1-A's Basic Hero Training Class and announces that the class will participate in Battle Trials. Eijiro is excited by the idea of combat training and changes into his hero costume with the rest of his class. Eijiro is partnered with Hanta Sero on "Team J" after being randomly paired.
While observing the battle between Katsuki Bakugo and Izuku Midoriya, Eijiro says Katsuki's unmanly for trying to ambush Izuku. He also questions what they're saying during the battle, as there's no sound through the surveillance footage. After witnessing Katsuki's large attack on Midoriya, Eijiro becomes increasingly concerned with Izuku's safety.
Watching the match between Shoto Todoroki vs Mashirao Ojiro and Toru Hagakure, Eijiro and All Might begin to shiver when Shoto uses his Quirk. Later on after the battle trials, when Izuku returns to class, Eijiro along with his other classmates surround him praising his performance during the Battle Trial and introduce themselves.
During their next homeroom class, Mister Aizawa announces Class 1-A must choose a class president. Eijiro and his classmates are excited by the idea of representing their class. After voting, Izuku Midoriya is elected and Eijiro openly supports him.
Later on, the school sirens go off at lunch and students evacuate in a panic, creating a large traffic jam. Eijiro is caught in the mob, but the situation is diffused by Tenya Iida, who reveals it is a false alarm caused by the media trespassing on school grounds. Afterwards, Midoriya resigns as class president and elects Iida in his place. Eijiro supports the decision because he saw that Iida manned up and took charge of the previous situation.
Shota interrupts and instructs the class to suit up and prepare to head out for rescue training. While on the bus traveling to an outside facility, Eijiro speaks with his classmates regarding Quirk's and their relation to pro-heroes popularity. He says that he wishes for a flashier Quirk, believing it would garner him more fame. Izuku argues that Eijiro's Quirk is strong enough to become a popular pro, but Eijiro restates his point by using Shoto and Katsuki as examples of Quirk's perfect for professional heroes.
The class eventually arrives at the Unforeseen Simulation Joint, greeted by Thirteen. But they are interrupted by the arrival of the League of Villains. As the villains keep showing one after another, Eijiro is seen evacuating with his class, but they end up coming across Kurogiri, who blocks their way. Eijiro, together with Katsuki, attempt to attack the villain but are unsuccessful due to Kurogiri's amorphous body. The villain then uses his Quirk and warps Eijiro, along with Katsuki, to the Ruins Zone of the U.S.J.
In the Ruins Zone, Eijiro and Katsuki are ambushed by a group of villains, but both are able to hold their own and defeat them. Eijiro says that they should go and save the others, worrying about his classmates that do not have battle-effective Quirks. Katsuki, however, decides to go and fight Kurogiri instead, much to Eijiro's surprise. After hearing Katsuki's reasoning to go after Kurogiri, Eijiro notices Katsuki's calm attitude, much to Katsuki's chagrin. Eijiro decides to go with Katsuki.
Eijiro and Katsuki eventually reach the central plaza, where they attack Tomura and Kurogiri respectively. Eijiro attempts to punch Tomura, but the latter evades him. Eijiro expresses his desire to support All Might, but the hero refuses and tells them to escape instead. Tomura attempts to attack Eijiro and the others, but All Might's fight with Nomu prevents him from getting close. Eijiro and the others watch in awe of All Might's strength as the Number One Hero defeats the Nomu.
Eijiro withdraws himself from the rest of the fight, but witnesses Izuku rushing to All Might's side. Izuku and All Might are both saved when Tenya returns to the U.S.J. with pro heroes as reinforcements. After the remaining villains are defeated, Eijiro rushes to check on Izuku's condition. Cementoss arrives and blocks Eijiro's way before he discovers All Might's weak form. He instructs Eijiro to regroup with his class. Eijiro follows the heroes instructions and joins his class outside the facility before returning to school.
Two days after the attack on the U.S.J., Eijiro and his classmates return to school and talk about the attack. After Mister Aizawa returns to class, Eijiro and his peers lectured about the importance of the upcoming U.A. Sports Festival. Eijiro is more than excited to participate despite some of his classmates concerns over holding the Festival soon after the attack. Eijiro is freaked out when Ochaco Uraraka gets overly excited and demands the class chant that they will do their best.
At the end of the day, Class 1-A's exit gets blocked by a crowd of students from other classes. Katsuki confronts and insults the crowd, making enemies out of everyone else before leaving. Eijiro tries to get Katsuki to defend the class, but he replies that the crowd of students are irrelevant and it's only important that he defeats them. Eijiro is moved by Katsuki's "manly exit".
For the next two weeks before the festival, Eijiro trains at the Replica City District. When the day of the U.A. Sports Festival arrives, Eijiro changes into his P.E. clothes and prepares with his class in their designated waiting room. Like the rest of his peers, Eijiro takes notice when Shoto Todoroki challenges Izuku Midoriya. He tries to diffuse the situation, but Shoto refuses him and replies that the festival isn't a team effort.
Before the festival begins, First Year Chief Referee: Midnight calls Katsuki to the stage to lead the pledge. Instead, he simply announces that he will win the games, angering his fellow competitors and generates booes from the crowd. Eijiro and his class aren't surprised, and knew Katsuki would say something like that. Midnight announces the preliminaries will be an Obstacle Race four kilometers around the stadium, which immediately begins after the rules are explained. Shoto gets out to an early lead by freezing the stadium's exit, but Eijiro avoids the ice by jumping through the exit. Next, everyone reaches the first obstacle: a blockade of Villain-bots from the entrance exam, including several giant Zero-Point Villains.
Shoto freezes one of the giant robots solid while it's off balance, causing it to fall on Eijiro and TetsuTetsu from Class 1-B. Eijiro survives by using his hardening Quirk and damns Shoto for endangering others like that. Eijiro believes only he could have survived, but is shocked when TetsuTetsu uses his Steel Quirk to break free from underneath the robot as well. Eijiro is discouraged when he notices TetsuTetsu's quirk is similair to his own, fearing he won't be able to stand now. Eijiro finishes the Obstacle Race in ninth place, qualifying him for the second event: The Cavalry Battle.
After Midnight explains the rules, everyone forms their teams for the Cavalry Battle. After failing to team up with Shoto, Eijiro asks Katsuki to form a team. Katsuki forgets Eijiro's name and greets him by "dumb hair". Eijiro takes exception to this and says their hair styles are similar. Eijiro understands that Katsuki will need a strong front horse for the Cavalry Battle and his Quirk would be perfect for resisting Katsuki's explosions. Katsuki replies that he needs someone with guts, and Eijiro convinces to form a team by promising to go after Izuku Midoriya's team together.
The Cavalry Battle begins and like many of the other teams, Team Bakugo goes after Team Midoriya's ten million points. Katsuki tries to attack them in the air, but is blocked by Fumikage Tokoyami's Dark Shadow Quirk. Fellow member of Team Bakugo, Hanta Sera catches Bakugo, garnering Eijiro's praise. Soon after, Team Bakugo's points are stolen by Team Monoma. Neito Monoma from Class 1-B taunts Katsuki, enraging him. An angry Katsuki tells Eijiro that their plans have changed: before they go after Team Midoriya, they must destroy Class 1-B.
Neito evades Katsuki's first attack and then blasts him with an explosion. Surprised, Eijiro comments that it's weird Neito is using Katsuki's Quirk. Then Neito quickly taps Eijiro's forhead before blocking a Katsuki's next attack. When the smoke clears, Eijiro is shocked to see Neito is using his hardening power to defend himself. Katsuki figures out Neito's Quirk is the ability to copy other powers.
Katsuki tries to attack again, but Eijiro is immobilized by Kojiro Bonto's Quirk and Neito flees. The next time Team Bakugo is able to get close, Katsuki flies off and attacks on his own. Although Eijiro warns him not to fly off on his own, Katsuki is able to steal enough points to move his team to second place. Eijiro is satisfied, but his team leader is anything but. Katsuki demands that they take the rest of Team Monoma's points before going after Izuku's team. Using Hanta and Mina's Quirk's strategically, Team Bakugo charges at Neito and Katsuki takes back their original headband. They go after the ten million points, but time runs out just as they arrive on the scene. Hanta is happy with second place, but Eijiro comments that Katsuki is not satisfied at all, as they all watch him scream out of anger.
Midnight announces the finals will be a One on One Fighting Tournament and everyone draws lots to see who they will face in the first bracket. Eijiro expresses his excitement to participate in the finals he has watched every year growing up. He matches up with Tetsutetsu, much to his irritation.
Eijiro duels Tetsutetsu in the seventh match of the first round. They receive similar intros from Present Mic, further annoying Eijiro. He and Tetsutetsu fist fight using their hardened bodies, trading blows back and forth until they eventually knock each other out. Midnight rules that after they both recover a victor will be decided via an arm wrestling contest.
Once the last match of the first round ends the arm wrestling match commences. They both use their Quirks to try and gain an advantage, but Tetsutetsu's iron gives way and Eijiro pins his arm down to win and move on to the next round. Eijiro offers his hand to Tetsutetsu, congratulating him on a match well fought. Tetsutetsu shakes Eijiro's hand and thanks him for the compliment.
Eijiro returns to his class just after the start of Izuku and Shoto's battle. He's upset at himself for arriving late, but Denki congratulates him on his victory. Eijiro thanks him and comments that Katsuki is next, to which he replies that he'll kill Eijiro. Eijiro laughs him off and responds by sarcastically complimenting Katsuki for having moves that blast the whole stadium. Katsuki tells Eijiro and Denki that Quirk's are physical abilities and they all have their limits.
In the last match of the second bracket, Eijiro faces off with Katsuki. Eijiro cuts Katsuki's cheek near the start of the bout and takes an explosion straight to his hip. Eijiro is unphased thanks to his Quirk's defensive ability.
He taunts Katsuki, telling "explosion boy" that his blasts will not work and goes on the offensive by trying to deliver a series of punches. Katsuki evades the strikes and is eventually able to injure Eijiro with another direct hit to his torso. Katsuki figures out that Eijiro's hardening ability weakens with every hit, and that he has been overusing it to keep his entire body hard throughout the fight.
Katsuki capitalizes and unleashes a flurry of explosions until Eijiro is ultimately overwhelmed and knocked unconscious. Eijiro is treated for his injuries and returns to the stands in time for Katsuki's next round fight against Fumikage Tokoyami. Fumikage ultimately surrenders and Eijiro surmises that his Quirk is weak against light, and Katsuki was the perfect person to exploit that.
Along with his class, Eijiro watches the closing ceremony of U.A.'s Sports Festival. He comments that Katsuki has been in a fury ever since he woke up after being knocked out at the end the finals.
Eijiro returns to class two days after the Sports Festival. Mister Aizawa teaches them about Hero Informatics and Eijiro fears the topic may be on hero laws. He's excited to learn that the class will choose their hero names for upcoming internships. Aizawa reveals the draft results prior to the festival and Eijiro received sixty-eight offers from Pro-Heroes.
Midnight joins the class to help them select hero names for their upcoming internships. Eijiro chooses the moniker Sturdy Hero: Red Riot. Midnight questions whether this name is a nod to an old school hero Crimson Riot, to which Eijiro confirms. Eijiro replies that although he's old school, Crimson Riot is his idol and he wishes to be just like him. Midnight advises him to live up to the name and Eijiro tells her that he will.
After Class 1-A is finished formulating their hero names, Eijiro is given his list of offers so he can choose a Hero Agency to intern at. He wishes to fight crime in a big city and elects to choose Fourth Kind's Agency. When first day of the internships arrive, Katsuki receives his costume and boards a train to Fourth Kind's agency.
Eijiro meets Tetsutetsu at the Fourth Kind's Office, surprised to see his newfound friend there. Fourth Kind explains that his office were able to extend two offers because he was impressed with the two young men's spirit. The Chivalrous Hero teaches them about hero wages from the government.
The next day, Eijiro and Tetsutetsu make lukewarm tea and are punished by Fourth Kind. He punches both of them with their Quirks active and says it is satisfying to hit their hardened bodies.
The workplace training comes to an end and Eijiro returns to U.A. In Class 1-A, Eijiro and Hanta laugh at Katsuki's new hairstyle, causing Katsuki to retaliate in anger.
After Foundational Hero Studies is finished, Eijiro is in the boys' locker room changing into his school uniform. He states that he needs to focus on his maneuverability.
One week before final exams, Aizawa reminds Class 1-A to study for their written and practical components before ending class. Many of the students opt to study with Momo, and Eijiro mentions how he should tag along. Irritated, Katsuki says that he'll beat the material into Eijiro's head himself. When Katsuki storms out of class later on, Eijiro mentions it's been a while since Katsuki has been that intense. A week passes and Class 1-A finishes the written section of the finals. They change into their hero costumes and join their teaches at the practical exam area. There, Principal Nezu announces the practical portion will involve pairs of students facing off against one teacher. Eijiro and Rikido are paired against Cementoss in the first bout. 
Eijiro and Rikido agree to attack Cementoss from the front because he's slow, but their opponent creates multiple cement walls to block them. Eijiro and Rikido use their Quirks to smash through the cement blocks, but Cementoss continues to make more. Before long, the tandems Quirks run out of steam and Cementoss defeats them using a cement attack, and as a result, they fail the practical.
Eijiro and Rikido mull over their defeat as the remaining matches are underway. Following the conclusion of final exams, Class 1-A returns to their homeroom. Eijiro, Rikido, Denki and Mina are all crestfallen that they are unable to go to training camp because they failed the practical. Izuku tries to cheer up those who failed, but Denki launches a comical tirade at Izuku. Soon after, Aizawa enters the classroom and informs Class 1-A that all of them are going to the training camp. Eijiro and the others are happily shocked to hear this, but they will all receive harsh extra studies for failing. Class 1-A decides to go to the Kiyashi Ward Shopping Mall to buy necessities for the forest lodge. Eijiro asks Katsuki to tag along, but he refuses. At the shopping mall, Eijiro sees that everyone has different needs and suggests splitting up and meeting up later.
The next day after the shopping trip, at U.A., Aizawa informs Class 1-A that the destination for their lodge trip has been changed and the new destination will only be announced on the day of the event. The first semester has come to a close and summer break has begun.
On the day of the event, Class 1-A boards a bus that will take them to the forest lodge destination. An hour later, Class 1-A's bus stops for a restroom break. However, Class 1-A notes that Class 1-B isn’t at the rest stop. Suddenly, two females wearing cat-like costumes and a small boy appear; the females excitedly introduce themselves as the professional hero team, the Pussycats. Mandalay explains to Class 1-A that they will be staying at the base of a mountain which is a part of their domain; they have three hours to reach the base of the mountain using their Quirks as they please and those who don't arrive there by 12:30 pm won't be served lunch. Soon, Class 1-A realize that they have already arrived at their destination and their training camp has already begun with the Pussycats being their supervisors. Some of Class 1-A try to leave, but Pixiebob uses her Quirk to create a landslide of dirt that causes Class 1-A to fall into a forest called Forest of Magic Beasts, which they must traverse to reach their destination. Suddenly, Class 1-A are confronted by a beast; Koji Koda tries using his Quirk to control the beast but it does not work due to the beast being made out of dirt. Izuku, Shoto, Tenya, and Katsuki mobilize and use their Quirks to destroy the dirt beast.
After Izuku, Tenya, Shoto, and Katsuki destroy the dirt beast, Class 1-A begins traversing the Forest of Magic Beasts. The journey through the forest to the Cabin takes around eight hours. Class 1-A arrive at the facility, battered and tired. It is now 5:20 pm in the evening. Pixiebob admits that she thought they would take longer to get to the facility; she praises Class 1-A for figuring out her Earth Beasts quickly and easily. Class 1-A fetch their luggage from the bus and put it in their rooms. Class 1-A go to the dining hall to have dinner. After finishing eating dinner, Class 1-A go to the hot springs. The next day at 5:30 AM, Class 1-A are outside the training camp cabin. Aizawa greets his students and tells them that they will undergo reinforcement training to strengthen and upgrade their Quirks which will also allow Class 1-A to obtain their temporary licenses.
Eijiro trains with his class. On the night of the third day, the League of Villains's Vanguard Action Squad attack the camp. During the attack, Eijiro is in the facility being protected by Blood King. A Dabi clone attacks the facility only for the threat to be dealt with quickly by Shota. Eijiro asks Aizawa to allow him to fight, but he orders him and the other students to stay in the facility. After the attack is over, the training camp ends and Eijiro returns home.
Two days later, Eijiro goes to the hospital to visit Izuku. Eijiro tells Izuku that they can save Katsuki thanks to a transmitter being planted on one of the villains by Momo and Yosetsu. Tenya deduces that Eijiro wants Momo to create a device for themselves so that Class 1-A can track down and rescue Katsuki themselves. Tenya demands that the rescue of Katsuki be left to the pro heroes, but Eijiro is tired of being unable to do anything that will make him a hero and a friend, causing them to get into an argument. After being calmed down by Denki and Tsuyu, although he fully aware that the matter should be dealt by professionals, Eijiro encourages Izuku to go through with the plan as it will allow him to reach and save Katsuki.
Most of Class 1-A are against the plan. After the doctor requests the class to leave, Eijiro tells Izuku that he and Shoto will be waiting outside the hospital tonight for him and Momo so that they can go and rescue Katsuki.
Eijiro teams up with Shoto, Izuku, Tenya and Momo to try and save Katsuki. As the rest of the team, he ultimately doesn't do much as he is paralyzed with fear by All For One and watches his battle with All-Might. However, when Izuku reveals his plan to save Katsuki, Eijiro proves his worth by being the key of the whole plan. Indeed, being the only one in 1-A that Katsuki holds some respect for, Eijiro could rescue Katsuki without hurting his pride and ultimately lighten the burden on All Might's shoulders. After that, along with the Bakugo-Rescue team and Bakugo himself, he watch as All Might struggles to win against his archenemy: All For One. He then goes home after bringing Katsuki to the police.
Eijiro and the rest of Class 1-A are charged with coming up with new special moves. He passes the exam.
He and Tamaki end up interning under Fatgum. He encounters a criminal with Quirk destroying bullets and defeats him.