Yui Kodai/Synopsis

U.A. Sports Festival Arc
Yui first appears at the opening ceremony for the U.A. Sports Festival. She takes the field with her fellow classmates in Class 1-B. Yui is able to qualify for the Cavalry Battle after taking 31st place in the Obstacle Race. For this event, Yui teams up with Kojiro Bondo and Manga Fukidashi, forming Team Kodai. It is later revealed by Neito Monoma that she was a part of the group of Class 1-B students who went along with his plan to hold back in the Obstacle Race so that they could observe Class 1-A's Quirks and tendencies.

As Team Bakugo and Monoma engage in a confrontation, Yui's team interferes with Kojiro using his Quirk to halt their attack. When Team Bakugo takes most of Team Monoma's headbands, her team goes after Team Monoma. However, Yui's team gets knocked out of the match by Team Monoma, leaving them with zero points. When the battle ends, Yui is unable to continue to the tournament.

Forest Training Camp Arc
On the day of the forest lodge trip, when Neito mocks Class 1-A, Yui is seen observing alongside Setsuna Tokage, Reiko Yanagi, Kinoko Komori, and Pony Tsunotori. When Class 1-B arrives at the training camp, their homeroom teacher explains that they will be extending their Quirks and that the more they use their Quirks, the stronger they will be by breaking their limits. When Class 1-B witnesses Class 1-A's training, Itsuka Kendo asks Shota Aizawa how they can observe everyone, and he explains that is the reason the Wild, Wild Pussycats are around.

Later that night, Class 1-B acts as the scaring group during the test of courage. In the forest, Itsuka and Juzo Honenuki commend Yui for scaring Shoto Todoroki and Katsuki Bakugo. However, smoke entering the forest knocks out Juzo. Itsuka takes action and uses her Quirk to cover Yui from the smoke. The two later encounter Tetsutetsu Tetsutetsu and an unconscious Ibara Shiozaki, in which Yui is told to stay behind to protect Juzo and Ibara as he and Itsuka head towards the source of the poisonous smoke. Yui and the other 1-B students are later rescued by Shota during the aftermath of the Vanguard Action Squad Invasion.

U.A. School Festival Arc
When Class 1-B encounters Izuku taking Eri on a tour through the School Festival, Neito tells the two of his class' play "Romeo, Juliet and the Prisoner of Azkaban: The Return of the Kings!" Yui is later seen performing during Class 1-B's act, playing the role of Juliet.

Joint Training Arc
Yui and the rest of Class 1-B show up for the Joint Training Battle, with Neito loudly claiming that the momentum is now moving to their side. Upon selecting teams, Yui is paired up with Neito, Reiko, Nirengeki Shoda, and Hitoshi Shinso for the final match. As the matches go on, Yui and her teammates watch Neito with exasperation as he shows further signs of insanity.

When their respective match begins, Class 1-B proceeds to initiate their plan of taking down Izuku Midoriya as their top priority. As Ochaco Uraraka and Minoru Mineta attempt to catch their enemies off-guard, Yui and her teammates (excluding Neito) use this opportunity to attack them with a combined attack. While sending multiple objects straight towards 1-A, Mina Ashido attempts to melt the flying objects with her Acid Quirk, only for Yui to reveal her Size Quirk by enlarging the metal bolts. Upon scattering 1-A, Team 1-B vows to track down the remaining members in hopes of capturing them for a chance at victory.

However, Class 1-B's plan gets interrupted when Izuku suddenly loses control of his Quirk, unleashing black tendrils that destroy multiple sections of the battlefield. Yui and her team notice Izuku rampaging across the battlefield. After Hitoshi and Ochaco manage to stop Izuku's out-of-control Quirk, Neito tries to ambush the two 1-A combatants. Neito's attempt fails, which leads to Reiko saving him from getting beaten down by Ochaco. Yui and Nirengeki follow up with Mina and Minoru attempting to ambush the opposing team. Yui takes action by summoning an enlarged shield to defend her teammates. With all participants together, the final match becomes an all-out brawl between 1-A and 1-B.

Mina starts off the skirmish by launching multiple shots of acid straight towards 1-B. Yui attempts to protect her classmates with another giant shield, struggling to hold her opponents off. However, Minoru and Mina perform a new combined special attack which involves sending the grape pervert bouncing around the battlefield. Yui and Nirengeki both express annoyance upon realizing the cleverness behind Minoru's plan. Despite this, Nirengeki manages to knock out the flying Minoru with a single punch, allowing Yui and Reiko to continue their assault on Mina.

As things become dire for Mina and Minoru, Ochaco arrives in the nick of time to restrain the remaining 1-B fighters. Yui then expresses shock as she watches Ochaco knock Reiko out. Before she can react, Ochaco quickly grabs Yui's arms and pushes her to a nearby wall with Minoru's balls, restraining her there. Yui and her classmates are present when Vlad King announces that Hitoshi is admitted into the Hero Department. In the evening, Class 1-B visits 1-A's dormitories for socializing and dinner. Yui is seen looking away from the party, not wanting to engage in a conversation.

Endeavor Agency Arc
With the start of the second Hero Works-Studies, she teams up with Juzo for their internships.

Paranormal Liberation War Arc
The months go by until it's late March. Thanks to the police investigation and its spy Hawks, the Hero Public Safety Commission uncovers the plans of the Paranormal Liberation Front and its allies. With this information, the Commission organized a large force of Pro Heroes, divided into several teams to take down the villain organization.

While a team of heroes raids the Jaku General Hospital, where the Nomu are created, another team does the same at Gunga Mountain Villa, the Front's main headquarters. Yui is assigned as a member of the Villa Backup Team along with some of her classmates and Class 1-A students. Their mission is to provide support to the heroes who raid the Gunga Mountain Villa and prevent villains from escaping. Despite the successful surprise attack in the villa, the plan is ultimately falling apart when Gigantomachia has started to move after Tomura Shigaraki has awakened in the Jaku General Hospital.

After picking up the League of Villains from the villa, Gigantomachia started to streak across the landscape in order to reach to Tomura, scraping and decimating many of the heroes who get in his way. With the Villa Raid Team still occupied at the villa dealing with the Paranormal Liberation Front, Momo received final words from Midnight and rallies everyone in Class 1-A and 1-B to prepare themselves to fight the rampaging behemoth and the League of Villains coming their way.

As Gigantomachia comes into their direction, Momo plans a trap for the heroes to setup before the behemoth arrives to their location. She gives each of her classmates a capsule full of anesthesia that is potent enough to knock the giant unconscious when they throw it into the mouth. Once the traps are set, the students detonate explosives deep in the mushy ground formed by Juzo Honenuki, halting the giant into sinking down. The students attack the giant and the villains, but none of them are strong enough to take the beast head-on.

Gigantomachia blows them away and Dabi incenerates the surrounding forest area that blocks the other students from getting closer, but Mina Ashido successfully runs through the flames thanks to coating her body with Acidman which nullifies the flames. She loses her capsule and was about to be crushed when Eijiro Kirishima pushed her aside to save her life. Once Eijiro manages to climb his way up to Gigantomachia, Yui and the U.A. students fire cannons created by Momo to distract the behemoth long enough to allow Eijiro to throw the anesthesia capsule into the mouth successfully. The rest of the heroes from the Villa arrive to assist the students.

However, things turn for the worse when Gigantomachia still managed to move on his own and grinds the heroes to dust, but Majestic rescues the students by using his Magic to levitate the students out of the way before he was killed. Gigantomachia has successfully broke free and rampages toward Jaku City to reach Tomura, as the students watch in horror of the situation growing even more worse than ever.

Final War Arc
With the heroes' numbers dwindling rapidly, the remaining heroes prepare themselves for the Final War against the villains led by All For One. While most of the Heroes successfully draw out the villains and split them up all over Japan, Yui and several other members of Class 1-A and 1-B are assigned to the Jaku Hospital Ruins, in order to protect Gigantomachia's containment facility, after he fell during the War.

Sure enough, a section of the Paranormal Liberation Front appear and attempt to free the beast, leading to a big battle. During the fight, they heard the news that Shoto Todoroki has defeated his oldest brother, Dabi, and that gives the heroes the push to keep on fighting.