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.
During their battle, Snipe corners Toru and Mezo behind stone pillars. Mezo admits escaping will be difficult because their only advantage is the weights Snipe is wearing. Snipe throws a smoke bomb and fills the area with smoke. Then, he moves towards them and shoots down the pillar they're hiding behind. Mezo moves to another pillar and asks Toru if she's okay, but realizes that she has disappeared. Mezo realize she's taken off her clothes and gone into full stealth mode. Understanding her intentions, Mezo decides to runs out to distract Snipe, and the teacher shoots at his feet before warning him not to be hasty. Mezo raises his hands and Snipe questions if he's giving up. Suddenly Toru handcuffs Snipe, taking him by surprise. After this 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.
At some point Mezo arrives to I-Island along with Fumikage.
On the night of the I-Island preview, Fumikage and Mezo our hanging out in the streets in the Business Area. A curfew announcement is made, so they rush back to their hotel room.
Mezo hangs out with the rest of his class after the I-Island Expo opens up.
Due to a shopping center incident, where Izuku had an encounter with Shigaraki, 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.
Following the end of the semester, Mezo, along the rest of Class 1-A boys, is invited by Deku to participate in an endurance training session, an idea of Minoru and Denki (an excuse to be able to see the girls in bikini). When Katsuki arrives and challenges Izuku, the boys opt to compete against each other by sprinting down the pool. Shota arrives to kicking all the students out of the pool area due to the expiration of their time limit.
On the day of the summer camp, they are informed that the new destination will be a mountainous region, owned by the professional Hero Team, the Wild, Wild Pussycats, where they will undergo reinforcement training to strengthen and upgrade their Quirks which will also allow Class 1-A to obtain their temporary licenses. Class 1-A’s bus stops for a restroom break, where they meet Pixie-Bob and Mandalay, who challenge the students to reach base camp past the forest in three hours. Then, all Class 1-A is forcibly thrown into Beast's Forest by Pixie-Bob's Quirk.
To reach their destination, they must traverse the forest, which is inhabited by dozens of hostile earth monsters created by Pixie-Bob. The Students work together to fight their way through the forest. To help his classmates, Mezo, along with Kyoka, use his Quirks to detect the location of the approaching beasts. The journey through the forest to the Cabin takes them around eight hours before finally arrive at the facility, battered and tired. Class 1-A fetch their luggage from the bus and put it in their rooms, and later go to the dining hall and the Pussycats serve dinner.
The next morning, 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. Mezo trains together with Toru to increase the strength of both their Quirk by having Mezo quickly create multiple Dupli-Arms to look for Toru while she uses her invisibility to further hide her presence.
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 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. Seeing that their situation is desperate, Mezo tells Izuku that he can either save Fumikage with him or save Katsuki by himself. Izuku choses other option.
Izuku's plan is to lure the rampaging Dark Shadow to Katsuki and Shoto location by creating decoys from Mezo's Dupli-Arms. They arrive to where Katsuki and Shoto are along with Dark Shadow in pursuit, with Mezo asking one of them to make light. Moonfish senses Mezo and Izuku's presence and sends one of his teeth to attack them, but he is easily defeated by the rampaging Dark Shadow. After this, Shoto and Katsuki use their Quirks to calm down Dark Shadow.
Now back in control, Fumikage thanks them 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. 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. However, Katsuki angrily refuses their help. 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 and Fumikage.
Shoto tries to stop Mr. Compress with his ice, but he avoids his attacks and escape. 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.
They execute the plan and the three fly at Compress at high speed and reach him and smash him onto the ground. However, they land 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 Mezo took the marbles from Mr. Compress. As Izuku and Shoto prepare to backup Mezo, Kurogiri arrives and blocks their path. 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, causing him to spit out the real marbles.
Izuku, Mezo and Shoto rush to grab the marbles, although Izuku succumbs to his injuries. Mezo and Shoto continue rushing towards the marbles; Mezo grabs the marble containing Fumikage, thus saving him, however Dabi snatches Katsuki's 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.
Due to the kidnapping of Katsuki, the students had to be transferred to Heights Alliance, a series of doors built by Nezu in order to protect the students. Shota meets with his students outside of the building. He states they must start preparing to get provisional hero licenses like they had planned to during the training camp. Before allowing Class 1-A students to enter in their new residence, Shota scold the entire class for having allowed five of them to rescue Katsuki without a license and did nothing to stop them. He further more says he would have expelled everyone but Katsuki, Toru and Kyoka, if it weren't for All Might's sudden retirement and asked them to stick to the rules from now on.
Class 1-A arrange their luggage in their respective bedrooms. Later that night, the boys of Class 1-A are in the common space on the first floor. The girls of Class 1-A show up and suggest a room showcasing competition. When they check out Mezo’s room, they are surprised because, apart from a desk and a futon, his room is practically empty. Shoto suggests that he is minimalist and Mezo explains that it's because he hasn't had any "worldly desires" ever since he was young.
The following day, Shota explain to Class 1-A that their next goal is to obtain their "Provisional Hero Licenses". To ensure success in the Provisional Hero License exam, they will develop their own ultimate moves, along with the guidance of Midnight, Ectoplasm, and Cementoss in Gym Gamma. During the days until the exam begins, Mezo constantly trains his Quirk to make it stronger. .
The Hero Qualification Exam finally starts. Mezo, along his classmates, arrives at the test location in the National Dagobah Arena, where he meets students from other hero schools like the Shiketsu High School and Ketsubutsu Academy High School. The first part of the exam involves thinning out the amount of people who can succeed by playing a game, where people put three marks on their body and are given six balls to throw at the other examinee's marks. Those whose marks have been hit are disqualified. Participants need to eliminate two students to move on to the next phase of the exam.
The room in which the students are in unfolds to reveal the arena, in which there are varying environments to battle in. As the exam starts, the rest of the schools all attack U.A. at once, however, Class 1-A is able to defend themselves thanks to their ultimate moves. Yo Shindo tells his fellow classmates that he will shatter their solid defense, and uses his Vibrate Quirk on the ground, unleashing a powerful earthquake that completely shatters the ground and and causing members of class 1-A to disperse.
Due to this attack, Mezo gets scattered, and has to defend himself from the attacks of the rest of the students while looks for his classmates. He regroups with Kyoka, Tsuyu and Momo. In the city area they enter a building after failing to find the rest of their classmates, and here they fall into an ambush prepared by Saiko Intelli, from Seiai Academy. Putting Saiko's plan into action, her classmates trap the four in a room, closing it completely and sealing off Mezo and Kyoka's Quirks that are good for detecting people. Then, they incapacitate Tsuyu by cooling the hallway down excessively using the air conditioning system, causing her to start hibernating due her Frog Quirk. To protect her from the cold, Mezo has to cradle Tsuyu with his limbs while covering her with a blanket created by Momo.
Momo comes up with a plan and creates four headphones and a large amp. Saiko readies her allies to storm the room but is surprised when her rival students counter with a high-frequency sonic wave attack, knocked out all of the Seiai students except Saiko. Class 1-A students leave the room to find Momo's plan was successful. However, Saiko hides behind the door and then locks herself and Momo back inside the room.
Momo buys enough time for Mezo to break down the door and Tsuyu uses her tongue to restrain Saiko. Saiko questions why they didn't just leave Momo to fail but Mezo, Kyoka and Tsuyu state class 1-A sticks together and they would never leave their friend behind. Impressed by the U.A. students, Saiko admits defeat.
Having knocked out at least enough Seiai students for all of them to pass, the four U.A. students head for the winner's waiting room.  There they met up with Shoto and discover that they're the only ones who have passed so far from their class. Minutes later Izuku, Ochaco, Hanta, Katsuki, Denki and Eijiro arrive. After awhile the rest of class 1-A pass the first qualifier test..
The qualifiers are informed of the imminent start of the second test, in which the students will have to save the largest number of fake victims in a given period of time. Mezo uses his Quirk to locate victims during the second test and carry the "injured" back to safety. At the end of the trial, Mezo is one of the students who passes the exam, receiving his provisional license.
The next day, Class 1-A attend the U.A. opening ceremony. After this, at classes, Shota talks about Hero Work-Studies, hero activities done off campus. Three days later, Shota introduces to Class 1-A the best students of U.A. who will teach them about Hero Work-Studies: Mirio Togata, Tamaki Amajiki and Nejire Hado, better known as The Big 3. Despite they reputation, they are actually all a bunch of easygoing eccentrics. Class 1-A get confused when Mirio Togata challenges them all to a fight.
At Gym Gamma, Mirio tells Class 1-A to attack him whenever and wherever they want. Mezo, like the rest of his classmates, attacks him, but Mirio completely wiping the floor with Class A without breaking a sweat, thanks to his Quirk Permeation. While Class 1-A recovers from their defeat, Mirio explains how his Quirk works and advises class 1-A to participate in the Hero Work-Studies, as it will help them improve their career to be heroes, just as it helped him become one of the best students in the U.A.
Weeks later, after the police raid at the base of Shie Hassaikai, Izuku, Eijiro, Ochaco and Tsuyu return to their dormitories at the school, where Mezo, along the entirety of Class 1-A, is checking to make sure his classmates are doing okay, after their mission.
Days later, in the classroom, Shota Aizawa announces that U.A. High School will be having a School Festival and tells Class 1-A to pick out a program to perform at the festival. All the students give their idea about what they should do for the festival, but they do not reach an agreement. Later at Heights Alliance, the entire Class 1-A decides that their school festival program will be a live performance and dance with party space. 
Class A-1 students are divided into three groups, Band team, Dance team and Staging team, Mezo is part of the second one. Until the festival, the dance team constantly practices the choreography devised by Mina.
On the appointed day, Mezo, along with the rest of Class 1, is preparing for the performance but get worried as Izuku is late to show up for the performance. Fortunately, Izuku arrives in time for the Class 1-A performance, which ends up being a huge success.
As November comes to a close, the Wild Wild Pussycats pay a visit to Class 1-A to celebrate Ragdoll's reinstatement.
Mezo, along with the rest of Class 1-A are pitted against Class 1-B as part of the Joint Training Battle. He is placed in the third round of match-ups, along with Tenya, Mashirao, and Shoto. They will face Class B's team of Pony Tsunotori, Sen Kaibara, Juzo Honenuki and Tetsutetsu Tetsutetsu at Round 3. As they head to the Ground Gamma, Mezo notices that Tenya is in a good mood and he replies that his brother has been doing well in his recovery. Mezo congratulates him for that good news.
The Class 1-A team goes to the position where the class 1-B team is located. Their plan is send Mezo out to scout and then use Shoto to attack them in a group. Shoto does just that and freezes all four of the Class 1-B students with a Giant Ice Wall. However, Juzo was able to predict their strategy, and with his Quirk Softening, he frees his teammates from Shoto's ice, and each one attacks each member of the rival team. Pony uses her Horn Cannon to attack Mezo and Shoto. She predicted Mezo would hide safely behind Shoto, so she sends out her remote-controlled horns to separate Mezo from Shoto and carry him away, pins him to a wall while uses other two horns to propel Tetsutetsu against Shoto. While fighting Tetsutetsu, uses a Wall of Flames to get rid of him. The intense flames can not dissuade Tetsutetsu, but force Pony and her horns to retreat, and Shoto tells Mezo to go after her.
Pony is riding her horns to distance herself from Mezo, while is chasing by him through the labyrinth. She fires her horns at him, but Mezo uses his own Quirk grab the horns. Pony turns to face him because she wants to help Tetsutetsu. She uses her Thunder Horn to try and capture Mezo but he counters with his Octoblow barrage. As Pony launches her horns in rapid fire succession, Mezo is able to destroy them at the same rate. In attempt to end the stalemate quickly, Pony tries to sneak a horn around Mezo's back but he spawns an eye behind his head and catches it and destroys the horn. He signals Mashirao and the tailed hero destroys Pony's horn with his Tornado Tail Dance attack. He says that if he can prevent her horns from firing off, then she is not much of a threat. Mezo tells Mashirao to take her to the prison while Mashirao tells him to go support Shoto.
They're both shocked when Juzo appears behind Mezo using his Quirk to soften the ground. Juzo sinks both under the ground and tries to force Mashirao to release Pony but Mashirao counters with his Fist of the Tail: Swamp Smack Spin Super Move. This helps Juzo and Mashirao get free but Pony uses her horns to send Mashirao into her team's prison, eliminating him from the battle. After this, Juzo goes to help Tetsutetsu in his fight against Shoto, while Pony faces Mezo.
After an intense fight, Shoto, Tetsutetsu, and Juzo are all unconscious, while Tenya is trapped under a softened tower. Pony uses her horns to pick up and carry away all the unconscious players, but she is chased by Mezo, who tries to strike down her. Because transporting so many people slows her down, Pony realizes that she can't carry Shoto to jail without Mezo stopping her, so she decides to levitate herself high above the field to be out of Mezo's reach. Although with this decision the only thing she gets is that the round ends the draw, Pony prefers this to give the rival team a chance to beat them. Mezo can not do anything but watch in dismay as time limit expires and the battle ends.
After this round, Mezo is dedicated to watch the two remaining rounds.