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 his 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 Bondo 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 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 Nirengki 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 evening, Class 1-B visits 1-A's dormitories for socializing and dinner. Yui is seen looking away from party, not wanting to engage in a conversation.