Kosei Tsuburaba/Synopsis

U.A. Sports Festival Arc
Kosei Tsuburaba first appears at the opening ceremony for the U.A. Sports Festival. He takes the field with his fellow classmates in Class 1-B. He is able to qualify for the Cavalry Battle after taking twenty-third place in the Obstacle Race.

For the second event, Kosei joins Team Monoma with Neito Monoma, Sen Kaibara, and Shihai Kuroiro. After stealing Team Bakugo's points, Sen urges Neito not to provoke Katsuki, because he believes this is something Katsuki would do.

As the Cavalry Battle's time limit runs low, Team Monoma plans to allow the clock to run out and secure second place. However, Katsuki suddenly flies at them using an explosion. Neito orders Kosei to stop Katsuki using his Solid Air Quirk, but the latter punches through it and snags two of their headbands.

Other teams try to capitalize on Team Monoma, but Kosei and Neito use his Solid Air Quirk to repel them. While fleeing, Kosei reminds Neito that they're still in fourth place despite losing two headbands. Neito agrees to defend their remaining points to the last, but Katsuki strategically uses his teammates' Quirk's to close the distance between the two teams. Kosei tries to stop him, but Katsuki blasts through the air shield and takes Neito's last headband. The Cavalry Battle ends, and without any points, Team Monoma fails to advance to the finals.

Forest Training Camp Arc
On the day of the forest lodge trip, when Neito Monoma mocks Class 1-A, Kosei is seen observing.

When Class 1-B arrives at the training camp. Homeroom teacher Sekijirou Kan 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. After Class 1-B witnesses Class 1-A's training, Itsuka Kendo asks Shota Aizawa if can they observe everyone, he explains that is the reason The Pussycats are around.

After the Vanguard Action Squad start their attack, Kosei is one of the first students to be affected by Mustard's Sleep Gas, being carried by Todoroki all the time while he and Katsuki are fighting Moonfish, and chasing Mr. Compress afterward. Shota eventually rescues Kosei and the other 1-B students during the aftermath of the Vanguard Action Squad Invasion.

Joint Training Arc
Kosei and the rest of Class 1-B show up for the Joint Training Exercise, with Neito loudly claiming that the momentum is now moving to their side. Upon selecting teams, Kosei pairs up with Ibara Shiozaki, Jurota Shishida and Hiryu Rin for Round 1. After Jurota incapacitates Tsuyu and Kirishima, Kosei uses his new special move, Air Prison, to trap Koji, rendering him completely defenseless. Kosei then climbs on Jurota's back and tells him to attack the rest. However it is shown that it was not Kosei speaking, it is Shinsou's new gadget, the Artificial Vocal Cords, that allows him to sound like anyone he wants.

Shinso then tries to use his Capturing Weapon on Kosei and Jurota but Kosei manages to trap Shinso with his airprison in time. Kosei then snaps Jurota out of Shinso's spell with a slap to the head and tells him to attack, while pondering about Shinso's quirk. Next Kaminari goes for an attack on the duo. Jurota answers by smacking Kaminari into a wall but got shocked by his Quirk but Kosei jumped off him in time to avoid it. However, Kosei is caught by Tsuyu in midair and sent straight to their jail. While in his prison cell, Kosei is excitedly thinking about how her he was almost entirely wrapped up in Tsuyu's tongue.

After Round 1 ends with Class 1-A scoring a 4-2 victory, Kosei and his teammates get scolded by Vlad King for having poor communication skills over deciding who's leader. Kosei later observes Round 5 with Manga Fukidashi, both mention that though Team 5-B have the odds on their side, most of them aren't capable of enduring the prowess of 1-A's heavy hitters. Kosei and his classmates are all present when Vlad King reveals that Hitoshi is admitted into the Hero Department. In the evening, Class 1-B visits 1-A's dormitories for socializing and dinner. Kosei engages in a conversation with his friend Sen and Denki as the two classes appear to be getting along just fine.