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 along with Neito Monoma, Sen Kaibara, and Shihai Kuroiro. After stealing Team Bakugo's points, Sen urges Neito not to provoke Katsuki because he believes their actions are similar to 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 Kosei's 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 end. However, Katsuki strategically uses his teammates' Quirks to close the distance between the two teams and goes for Neito's last headband. Kosei tries to stop him, but Katsuki blasts through the air shield and takes the headband.
The Cavalry Battle ends, and without any points, Team Monoma fails to advance to the finals.
On the day of the forest lodge trip, Kosei is seen observing as Neito Monoma mocks Class 1-A. When Class 1-B arrives at the training camp, their homeroom teacher Vlad King 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. The Wild, Wild Pussycats appear to explain that they will be there to help and guide them in their training.
On the 3rd night of the training camp, Class 1-B acts as the scaring group during the "Test of Courage". However, the Vanguard Action Squad arrive to interrupt the fun by invading the Beast's Forest in search of Katsuki. Kosei is one of the first students affected by Mustard's Gas. He is carried by Shoto while he and Katsuki are fighting Moonfish, until he is defeated by Dark Shadow.  
With Kosei still unconscious on his back, Shoto and other students escort Katsuki but during the walk he and Fumikage are captured by Mr. Compress. Shoto leaves the care of Kosei to Ochaco and Tsuyu while he, Izuku, and Mezo chase after Mr. Compress. Shota eventually rescues Kosei and the other 1-B students during the aftermath of the Vanguard Action Squad Invasion.
Kosei helps his classmates to prepare their theatrical performance "Romeo, Juliet and the Prisoner of Azkaban: The Return of the Kings!" for the U.A. School Festival. However, it seems that he does not participate in the actual play.
Kosei 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, Kosei pairs up with Ibara Shiozaki, Jurota Shishida, and Hiryu Rin for Round 1. After Jurota incapacitates Tsuyu and Eijiro, Kosei uses his new special move, Air Prison, to trap Koji and render 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 but rather Hitoshi's new gadget, the Artificial Vocal Cords, which allows him to sound like anyone he wants. 
Hitoshi then tries to use his Capturing Weapon on Kosei and Jurota, but Kosei manages to trap Hitoshi with his air prison in time. Kosei snaps Jurota out of Hitoshi's spell with a slap to the head and tells him to attack as he ponders about Hitoshi's Quirk. Denki goes for an attack on the duo, and Jurota answers by smacking him into a wall. He gets shocked by Denki's 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 excitedly thinks about how 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 their leader.  Kosei later observes Round 5 with Manga Fukidashi, with both students mentioning that although Team 5-B has the odds on their side, most of them aren't capable of enduring the prowess of 1-A's heavy hitters.
The Joint Training Battle ends with victory for class 1-A. In the evening, despite the fierce competition and the result, Class 1-B visits 1-A's dormitories for socializing and dinner. Kosei engages in a conversation with his friends, Sen and Denki. The two classes appear to be getting along well.