Sen Kaibara/Synopsis

U.A. Sports Festival Arc
Sen comes in 22nd place during the Obstacle Course. For the Human Cavalry Battle, he teams up with Neito Monoma as the rider, Kosei Tsuburaba, and Shihai Kuroiro. Unlike the rest of Neito's teammates, Sen is the only one to call him out for taunting Katsuki, explaining that the latter would've made the same immature move. Despite all their efforts, Team Monoma loses all of their headbands to Team Bakugo, which prevents Sen and the rest of his team from advancing to the tournament.

Forest Training Camp Arc
Sen is present along with the rest of his classmates when Vlad King instructs them on Quirk Training upon their arrival at Beast's Forest. When Class 1-B observes Class 1-A pushing beyond their limits, Tiger decides to call any simple power augmenting Quirks to go to him, in which Sen and Jurota Shishida dread at the sight of seeing Izuku training to the extreme. Sen notes that Tiger is rather scary in terms of appearance and demeanor.

Later that night, Class 1-B acts as the scaring group during the test of courage. However, the Vanguard Action Squad interrupt the fun by invading the forest in search of retrieving Katsuki Bakugo. Sen is one of the few who falls victim to Mustard's poisonous gas, the others being Kyoka, Toru, and most of Class 1-B.

Joint Training Arc
Sen and the rest of Class 1-B are all present for the Joint Training Battle, with Neito loudly claiming that the momentum is now moving to their side. Upon selecting teams, Sen is teamed up with Tetsutetsu Tetsutetsu, Pony Tsunotori, and Juzo Honenuki. Upon entering the new battlefield, Sen calls out Tetsutetsu for charging directly at their opponents without thinking about the flaws of his plan. Juzo then tells his teammates that Tetsutetsu's strategy plays right into their abilities.

The battle starts as Shoto Todoroki unleashes a giant ice wall at the opposing team, in which Juzo retaliates by turning it into liquid substance with his Softening Quirk. This allows Team 3-B to advance towards Team 3-A. Sen then activates his Gyrate Quirk to fight against Mashirao Ojiro. Sen appears to be gaining upper hand as his drill attacks keep Mashirao at a heavy disadvantage. However, his attack is quickly halted by Tenya Iida, who swiftly drags him towards Team 3-A's prison cage. Sen gets upset over Ingenium interrupting his "one-on-one fistfight" with Mashirao, in which the latter reminds him that he should treat the exercise like as if it was an actual battle in reality.

Tenya manages to successfully send Sen to Class 1-A's prison cage. However, Sen remains defiant until the end by continuously attempting to break free of Tenya's grasp, which heavily slowed down his opponent's movements. Vlad King later announces Sen as the MVP of Round 3, much to the latter's annoyance. When the match ends, it turns out that Sen and Mashirao are the only ones that were actually captured.

Sen is later present with the rest of his classmates when Hitoshi Shinso is finally admitted into the Hero Department. In the evening, Class 1-B visits Class 1-A's dormitories for dinner and spending time with one another. Sen is shown chatting with his friend Kosei and Denki Kaminari as the two classes appear to be getting along.