Sensoji/Synopsis

History
More than fifteen years ago, Sensoji attended U.A. High school, being a student in class B. During this time, he developed antipathy towards Shota Aizawa, whom he considered weak.

School Days Arc
In the summer of his second year at the U.A., Sensoji and his classmates participate in joint practical exercises with those of Class 2-A, a class to which Shota, Hizashi and Oboro belong. During the Battle Training, after confronting and defeating Shota, he criticizes him because he makes him shine less in battle, making fights against him "a snoozefest". Sensoji doesn't hesitate to tell Shota that his Erasure Quirk is useless and that he should go home instead of wasting others' time. Class 2-A homeroom teacher has to intervene to put a stop to Sensoji behavior as soon as he notices.

For the Hero Work-Studies, he is assigned to a battle-oriented agency, the Buster Union, coinciding with Hizashi, one of Shota’s friends. He gets from him the idea of wear shades, in order to shield his eyes from his own bright Blasts to help him aim his shots better. After a period of practices, all students returned momentarily to U.A. High School to show their progress, and Class 2-A homeroom teacher organizes a Two on Two Battle Royale Training at Ground Beta.

While waiting, Sensoji notices Shota has added Oboro's signature googles to his hero costume to help manage his Quirk, and gets upset because he assumes Shota is copying the shades he wears as part of his costume for a similar reason. Hizashi protests telling that Sensoji copied his sunglasses shtick. Shota just ignores Sensoji's provocations. At Oboro's suggestion, the teacher ordered to team up prioritizing the partnership during the Work-Studies. Hizashi protests because that means he had to team up with Sensoji, but the teacher ignores his complaints.

The battle begins, and despite Hizashi's grumbles, he makes a good combo with Sensoji, defeating their opponents without problems. At the end, Sensoji-Hizashi team faces the Shota-Oboro team. Before the two duos go against each other, Sensoji challenges Shota to face him, and says that if he wins Shota he must lose his goggles, because they bother him. Shota doesn't care about his rival's mania, but Oboro accepts the challenge, proclaiming that they wager their goggles while Sensoji and Yamada must ante up their shades.

Letting himself be carried away by his antipathy towards Shota, when the battle start Sensoji orders Hizashi to stand back, as he decided to face Shota and Oboro alone. Sensoji shoots his blast but they dodge the attack. Using his Quirk, Oboro manages to disorient Sensoji, allowing Shota to stand behind him and nullify his Blast Quirk with his Erasure. Taking advantage of the situation, Oboro knocked down Sensoji with his quarterstaff, causing him to lose his shades.

Despite the blow, Sensoji wants to continue, but Shota stops him, arguing that a two-on-one is not a fair fight and tried to return to him his shades. Sensoji can not stand the defeat and slaps Shota's hand away before smashing his own shades and grumpily stomping out of the class, ensuring that he does not need it and he didn't lose. The homeroom teacher compliments Shota and Oboro for their teamwork and admonishes Sensoji for his behavior.

A week later, the students return to their respective agencies to continue with the Work-Studies, and accordingly, Sensoji and Hizashi return with the Buster Union. The team of heroes is notified that a villain known as Garvey is causing great destruction in the city, and the entire Buster Union team, including the two students, flock to face him.

While flying over the city on four-rotor drone, Sensoji boasts of his new goggles, so Hizashi calls him copycat and reminds him what he said to Shota last time. Sensoji is bothered to hear about Shota, since he considers him a weakling. Hizashi defends his colleague and tells Sensoji that, while Shota overthinks things and a slow start, he is a tough opponent when he digs deep and finds his guts. Sensoji replies that he should stop losing his guts, which Hizashi admits is a fair point before calling him simple minded. The team leader orders them to stop the chit chat, as they are about to reach their destination.

Once there, the team leader orders everyone to unleash the full power of their Quirks. All of them fire powerful attacks at Garvey, but to their surprise, the villain absorbs all the attacks with his Stock Quirk. Before the heroes can react, the villain strikes back using their own attacks against them, with even greater power. The wave of energy swept a large area, and Sensoji and the entire Buster Union team is defeated.

The villain continues to wreak havoc in the city until he encounters Team Purple Revolution, a team of which Shota Aizawa and Oboro Shirakumo are members. During the confrontation, Oboro is trapped under a pile of debris but, in the end, Shota is able to defeat Garvey alone. Shota is being praised for defeating the villain alone, but he that insisted his friend Oboro cheered him on during the entire fight through his speaker. However, Sensoji explains that he could not hear anything from the speaker since it is completely busted, telling him it was likely he was giving himself a pep talk and imagine it was Oboro.

Shota brushes Sensoji‘s comment off and looks over the fallen rubble where Oboro was knocked out, only to find out his friend in a body bag, stained with blood, and finally realizes the terrible truth: Oboro Shirakumo died when debris fell on him. Sensoji continues his speech but Hizashi asks him to ease up, but Sensoji encourages Aizawa to be proud of himself for taking down Garvey all by himself, without anyone's help. Hizashi ends up telling Sensoji that he envies his simple mind for not realizing the point. Sensoji returns to recover, while Shota and Hizashi decide to stay a while longer in the place, mourning over the loss of their dear friend.

This event affected all its participants. In the school year following Oboro's death, Sensoji becomes a much nicer person, respecting Shota and no longer antagonizing him. In a training session against class 3-A, He is defeated by Shota in close combat, and tells him in what aspects he has to improve. Far from getting angry as he did before, Sensoji thanks him for the advice, accepting his hand to pull himself up. Sensoji tells him he isn't much of a brawler but support with his Blast Quirk, so he doesn’t consider this a real lost. Shota doesn't care about his bravado.

Time passes until he finally graduates from the U.A. His mother can't contain her excitement, much to Sensoji's embarrassment.