At some point, something happened to Mezo that caused him to start wearing a mask across his face. What this event was has not been stated.
Mezo makes a brief appearance during the Entrance Exam, using his Quirk from the top of a building and monitoring his surroundings.
Mezo is seen participating in the Grip Strength Trial, where he excels with a grip of 540 kilograms. Hanta Sero comments that Mezo brings to mind an octopus.
Mezo is paired off with Shoto Todoroki for the Battle Trial, where they get to face Mashirao Ojiro and Toru Hagakure. Due to Shoto's power, Mezo's role in the trial ends up being rather minor, using his Quirk to locate their opponents and then leaving the building under Shoto's request, who proceeds to freeze the entire building and recover the warhead by himself, giving them the win.
During the League of Villains' invasion at the Unforeseen Simulation Joint, Mezo and his classmates are faced with Kurogiri, who warps most of the class to different points of the building. Mezo is able to avoid this though, managing to protect Hanta Sero and Mina Ashido from being warped as well. After Kurogiri's attack, Mezo is able to confirm to Thirteen and Tenya Iida that all warped students are still in the simulation joint before they start confronting the warping villain himself.
As Tenya attempts to escape the building in order to call for reinforcements, Kurogiri tries to prevent him from leaving. Mezo manages to protect Tenya from one of Kurogiri's attacks, and thanks to the combined efforts of Ochaco Uraraka, Hanta, and Rikido Sato, Kurogiri is successfully warded off as Tenya manages to run away unscathed.
Mezo places 14th in the Obstacle Race, allowing him to participate in the next event, the Human Cavalry Battle. Minoru asks him if he could team up with him, which he allows after hearing his plan.
After the Human Cavalry Battle starts, Mezo and his team attack Izuku's team, carrying Minoru Mineta and Tsuyu Asui on his back while hiding them with his tentacles. After Minoru orders Mezo to attack, Mezo begins charging at Izuku and Shoto's teams, but Denki releases a huge amount of electricity, electrocuting Mezo and his teammates and become stunned as a result, unable to move. Shoto then freezes Mezo's legs, stopping him in his tracks. After the Human Cavalry Battle ends, Mezo and his team fail to qualify for the final event.
Mezo along with his class watches the closing ceremony of U.A.'s Sports Festival.
Two days after the Sports Festival, Mezo is in class. During the Hero Informatics period, Class 1-A has to decide on Hero names. Mezo writes down and reveals his Hero name: Tentacle Hero "Tentacole".
After Class 1-A is finished formulating their Hero names, Mezo is given a list of 40 workplaces in order to choose a workplace that he wants to train at. On the day of the workplace training, Mezo is at the train station with his class so that he can travel to the workplace of his choice by train.
Back in class, Izuku reveals the content of the exercise test to the class, much to Denki and Mina's happiness; Mezo notes that they are happy because they do not have to adjust their Quirks for real-life opponents. On the day of the exercise test, Mr. Principal reveals that Class 1-A will be fighting against U.A.'s teachers for their exercise test. Mezo is paired with Toru and they must face Snipe in their test exam.
Mezo, Toru, and Snipe arrive in an uninhabited city where their test exercise will take place. Snipe explains the 30-minute test; Mezo and Toru must either handcuff him or one of them must escape the battlefield within 30 minutes in order to win and pass the test. The test begins.
After a difficult battle against Snipe, Mezo along with Toru pass the practical test.
Back in class at U.A., Mezo learns that he passed the written test and will go with his classmates to the forest lodge which is a boot camp. Mezo along with his class go to Kiyashi Ward Shopping Mall to buy necessities for the forest lodge.
The next day after the shopping trip, at U.A., Shota informs Class 1-A that the destination for their lodge trip has been changed and the new destination will only be announced on the day of the event. The first semester has come to a close and summer break has begun.
On the day of the event, Class 1-A boards a bus that will take them to the forest lodge destination. An hour later, Class 1-A's bus stops for a restroom break. However, Class 1-A notes that Class 1-B is not at the rest stop. Suddenly, two females wearing cat-like costumes and a small boy appear; the females excitedly introduce themselves as the professional Hero Team, The Pussycats. The black-haired female explains to Class 1-A that they will be staying at the base of a mountain which is a part of their domain; they have three hours to reach the base of the mountain using their Quirks as they please and those who do not arrive there by 12:30 pm will not be served lunch. Soon, Class 1-A realize that they have already arrived at their destination and their training camp has already begun with the Pussycats being their supervisors. Some of Class 1-A try to leave, but Pixiebob uses her Quirk to create a landslide of dirt that causes Class 1-A to fall into a forest called Forest of Magic Beasts, which they must traverse to reach their destination. Suddenly, Class 1-A are confronted by a beast; Koji tries using his Quirk to control the beast but it does not work due to the beast being made out of dirt. Izuku, Shoto, Tenya and Katsuki mobilize and use their Quirks to destroy the dirt beast.
After Izuku, Tenya, Shoto and Katsuki destroy the dirt beast, Class 1-A begins traversing the Forest of Magic Beasts. However, the journey through the forest to the Cabin takes around about 8 hours. Class 1-A arrive at the facility, battered and tired. It is now 5:20 pm in the evening. Pixiebob admits that she thought they would take longer to get to the facility; she praises Class 1-A for figuring out her Earth Beasts quickly and easily. Class 1-A fetch their luggage from the bus and put it in their rooms. Class 1-A go to the dining hall to have dinner. After finishing eating dinner, Class 1-A go to the hot springs. Mezo does not appear to participate in bathing. The next day at 5:30 AM, Class 1-A are outside the training camp cabin. Shota greets his students and tells them that they will undergo reinforcement training to strengthen and upgrade their Quirks which will also allow Class 1-A to obtain their temporary licenses.
Mezo along with his class undergo training to improve their Quirks.
On the night of the third day of training, Mezo is paired with Fumikage for the test of courage. During the test of courage, the League of Villains Vanguard Action Squad begin their attack by filling the forest with poisonous gas. One of the villain's squad members called Moonfish ambushes Fumikage and Mezo. Mezo covers for Fumikage but loses one of his tentacles in the process. They manage to retreat from Moonfish. However, the ambush causes Fumikage to lose control of his Quirk. Mezo saves Izuku from Dark Shadow's claw and explains the situation to him. They turn their attention to Fumikage; Fumikage has been restrained by Dark Shadow, who is now larger and more ferocious due to the power of darkness. The empowered and berserk Dark Shadow begins taking control of Fumikage's body. Struggling, Fumikage pleads Izuku and Mezo to run away from him otherwise they will die. Izuku watches in shock.
The rampaging Dark Shadow attacks and destroys several trees around it. Dark Shadow attacks Mezo and Izuku, but Mezo is able to dodge in the nick of time. After maneuvering behind a tree, Mezo sees that Dark Shadow attacks anything that moves or makes, while also noting that Fumikage's regret and indignation is intensifying Dark Shadow's control over Fumikage as well as its own power. Fumikage pleads Mezo and Izuku to save the others and forget about him. Mezo informs Izuku that they need light to calm Dark Shadow down so that Fumikage can regain control; he tells Izuku that he can distract Dark Shadow while he runs back to the facility for help; Izuku is indecisive due to wanting to save Katsuki as well. Before Izuku can reply, Dark Shadow strikes at them, but Mezo is able to evade. Seeing that their situation is desperate, Mezo tells Izuku that he can either save Fumikage with him or save Katsuki by himself. Izuku apologizes to Mezo because he cannot make that decision and says that he has a plan. Mezo and Izuku arrive to where Katsuki and Shoto are along with Dark Shadow in pursuit, with Mezo asking one of them to make light. Before he can explain further, Moonfish senses Mezo and Izuku's presence and sends one of his teeth to attack them. The rampaging Dark Shadow uses its enormous claw to crush Moonfish, destroying Moonfish's teeth and knocking him out. Shoto and Katsuki are surprised at what happened. Izuku's plan was to lure the rampaging Dark Shadow to their location by creating decoys from Mezo's Dupli-Arms which worked. Izuku is happy to see Katsuki, glad that he does not have to save only one of his classmates and sacrifice the other.
After the rampaging Dark Shadow effortlessly defeats Moonfish, Shoto and Katsuki use their Quirks to calm down Dark Shadow, returning Fumikage to normal. Now back in control, Fumikage thanks Katsuki and Shoto for saving him. Mezo is glad that Fumikage is fine. However, Fumikage is regretful that he could not conceal his anger when Mezo lost one of his arms, which caused him to release Dark Shadow out of anger and lose control due to being enraged and spurred on by Dark Shadow's own rage. Fumikage is guilty for harming Mezo; Mezo tells Fumikage that he does not need to feel guilty and shows no ill will towards Fumikage.
Izuku tells Fumikage that their top priority is to protect Katsuki, who is the target of the villains. Izuku decides that they should escort Katsuki back to the facility and take a shortcut through the forest; Izuku thinks that with the power of Mezo's scouting capabilities, Shoto's powerful Quirk and the unrivaled power of Fumikage's Dark Shadow, escorting Katsuki will be a cinch. However, Katsuki is angry that he needs protecting and refuses their help. The Bakugo Escort Squad (consisting of Mezo, Shoto, Izuku, and Fumikage) begin escorting a begrudging Katsuki back to the facility. The Bakugo Escort Team travels to the facility and comes across Ochaco and Tsuyu. When Ochaco and Tsuyu notice that Katsuki is not with them, Izuku, Mezo, and Shoto turn around and see that not only Katsuki has disappeared but Fumikage as well. After the masked magician-like villain reveals to be the one who took Katsuki, Izuku demands that he give Katsuki back.
After handing Kosei to Ochaco, Shoto attempts to stop Mr. Compress by trying to freeze him with his ice attacks, but Compress's speed allows him to avoid Shoto's attacks. Mr. Compress also reveals that he captured Fumikage as well due to being impressed with his victory against the powerful Moonfish and states that he will be very useful, much to Izuku's anger. Compress begins escaping. Izuku hatches a plan to save Fumikage and Katsuki; Izuku asks Ochaco to make him, Shoto and Mezo float, then tells Tsuyu to use her tongue to throw their weightless bodies as hard as she can towards the magician, which will allow the three of them to fly towards the villain with Mezo using his arms to guide their trajectory; once they are in reach of the villain Ochaco will deactivate her Quirk. However, Ochaco and Shoto see that Izuku's injuries are severe and think he should stay put. Izuku insists that he goes along which convinces Ochaco and Shoto. They execute the plan; Tsuyu wraps her tongue around Izuku, Mezo and Shoto while Ochaco uses her Quirk to make them weightless. Tsuyu throws the weightless Izuku, Mezo and Shoto towards Mr. Compress. The three fly at Compress at high speed and reach him. Ochaco deactivates her Quirk, causing them to land on Compress and smash him onto the ground. Izuku, Mezo, Shoto and Compress landed right at the Vanguard Action Squad's rendezvous point. After apprehending Mr. Compress, Izuku, Shoto and Mezo see that they are right in front of Dabi, Himiko and Twice.
Dabi launches his flames which hits Mezo's arm and injures it. After Himiko attacks Izuku, Mezo smacks her away from Izuku. Mr. Compress prepares to give Dabi the captured Fumikage and Katsuki, only to find that the marbles are not in his pocket. Mezo reveals that he took the marbles from Mr. Compress. As Izuku and Shoto prepare to backup Mezo, Kurogiri arrives and blocks their path. After Himiko, Twice and the Nomu escape, Mr Compress reveals that the marbles Mezo took are not the captured targets and are decoys instead; the marbles Mezo took transform into ice, which shocks Mezo. Mr. Compress removes his mask and sticks out his tongue which has the actual marbles. Having overheard what is happening; Yuga takes action and fires his Navel Laser at Mr. Compress's face, which breaks Compress's mask and causing Mr. Compress to spit out the marbles containing Fumikage and Katsuki.
Izuku, Mezo and Shoto rush to grab the marbles, although Izuku starts succumbing to his injuries which causes him to fall to the ground. Mezo and Shoto continue rushing towards the marbles; Mezo grabs the marble containing Fumikage, thus saving him. Shoto is almost in reach of the marble containing Katsuki and nearly grabs it, but Dabi snatches it away before Shoto can grab it.
Mr. Compress deactivates his Quirk, freeing Fumikage and Katsuki. Mezo, Shoto, Izuku and the freed Fumikage see that Katsuki has teleported away with the Vanguard Action Squad.
After the Vanguard Action Squad leave, the training camp ends and Mezo returns home. Two days later, Mezo visits Izuku in the hospital and hears about Eijiro and Shoto's plan to rescue Katsuki. As Tenya orders them to stop thinking about such an attempt, Mezo stops Tenya. Mezo understands Eijiro and Shoto's frustration in not being able to save Katsuki and advises that they should not make the decision based on their feelings. Mezo leaves Izuku's room after the doctor requests them to.
At U.A. High School, Class 1-A along with Shoji choose to make a band and dance performance for the U.A culture festival.