Tamaki transferred to his new school during the spring, his first day as a third grader. Tamaki could not give a proper self-introduction and after the opening ceremony where they moved into the classrooms and formed groups, Tamaki was not able to insert himself into any group because he lacked the courage. Due to considering himself a boring person, Tamaki was not able to make any friends. Suddenly, Mirio Togata approached him after realizing that Tamaki was trying to say that he likes Heroes; it was this recognition that began the friendship between Tamaki and Mirio.
In middle school during a Quirk Education class, Tamaki tried manifesting something but only managed to manifest a tiny plant on his hand due to his nervousness. Meanwhile, Mirio tried phasing through a wall but only managed to phase his hands through it. Some of Mirio's classmates laughed at Mirio's struggles, but Mirio was not disheartened at all and resolved to pull off his phasing properly next time. While walking home, Tamaki praised Mirio's optimism while lamenting his own lack of self-esteem. Mirio cheered up Tamaki and said that he will shine brightly like the sun and the reason he can give it his all is due to Tamaki being there for him. Although he was nervous, Mirio praised Tamaki's resolve to never give up which inspired him to do the same. Despite Tamaki's low opinion of himself, Mirio always thought of Tamaki as a talented person who is cool and super fun and sunny to be around with.
After middle school, Tamaki and Mirio entered U.A. High School and they, along with Nejire Hado, eventually worked their way to the top, becoming The Big 3 of U.A. Tamaki chose his hero name to be Suneater, a name that was inspired after Mirio told him that he would outshine the sun.
Tamaki, Mirio, and Nejire visit Class 1-A and explain about the hero internships. When Shota Aizawa asks them to introduce themselves, Tamaki is overcome with anxiety and struggles to imagine Class 1-A as potatoes. Eventually, he gives up trying and turns around. As Mirio challenges the whole class, Tamaki warns Mirio to hold back so he will not completely frustrate his enemies when they lose.
As Class 1-A marvels at Mirio's Quirk, Tamaki thinks to himself how inexperienced they are for not recognizing all the hard work Mirio needed to refine and master his Quirk. Nejire points out to Tamaki that Mirio has gotten very strong, only for Tamaki to reply that he has been strong since he was a child but that he needs to learn to go easy on amateurs. After the battle, Class 1-A is frustrated because they did not know how to fight his Quirk. Nejire wants to explain how his Quirk works but Tamaki tells her that it is Mirio's time to speak.
In the Kansai Region in the Esuha Market, Tamaki Amajiki and Eijiro Kirishima are patrolling the streets with BMI Hero, Fat Gum. Fat Gum tells Tamaki that they need to fix his terrible mentality of his since he has a lot of talent, which stresses Tamaki. Tamaki thinks that the only reason Fat Gum scouted him was that he wanted to torment him, although Eijiro reassures Tamaki that Fat Gum was trying to encourage him. Tamaki states that because he is not one of those optimistic Heroes like Mirio, to which Eijiro replies that he is not like that either, and there have been many desperate times where he could not do anything and he wants to close the gap so that he can stand amongst his fellow classmates.
Suddenly, a fight breaks out. A thug is attacking another group of thugs for doing business on his turf. The group of thugs manages to get away, only for them to land into Fat Gum. The thugs start submerging into Fat Gum's body. However, one of the thugs manages to escape from Fat Gum's body using a Quirk. The thug starts running away but is caught by Tamaki's octopus tentacles. Tamaki uses his octopus fingers to reel the thug in and slashes the thug through the face with his clam hand, which defeats the thug. In the crowd of civilians, there is a man who takes out a gun, wanting to help his brothers which are the group of thugs. Fat Gum takes notice of this and yells at the crowd to get down.
The man fires the gun that releases a special bullet which hits Tamaki. The man fires another bullet, but Eijiro protects Tamaki and is hit instead. Fat Gum rushes to Eijiro and Tamaki's aid. However, Eijiro activated his Hardening before the bullet hit, which caused the bullet to simply bounce off him, much to the man's shock. Angry, Eijiro yells out that he will capture the man.
Tamaki manages to stand up with no injuries from the bullet, which shocks the man who fired the bullet. Tamaki attempts to capture the criminal who fired the bullet but is not able to activate his Quirk. The man begins running away while Eijiro and Fat Gum chase after him. Fat Gum orders Tamaki to stay and guard the area while Tamaki replies that while he is not injured, he cannot activate his Quirk, which surprises Fat Gum. Tamaki, Eijiro, and Fatgum manage to defeat the thugs.
Later on, Tamaki, Eijiro, and Fat Gum attend Sir Nighteye's hero meeting about the yakuza member Chisaki.
Tamaki joins the heroes in their mission to save Eri from the Yakuza. As the Heroes head down towards the basement but are surprised to see a wall before them. Mirio Togata uses his Quirk to phase through the wall, with Tamaki Amajiki explaining that Mirio's Hero Costume is made with special fibers from his hair, allowing his clothes to phase with him. Mirio has phased through the wall and sees that the path stretches on like Nighteye said. Mirio returns and tells the Heroes that the wall was thrown in the way to block them; Lock Down suspects that it was Chisaki's Quirk that put the wall there in the first place. Izuku Midoriya activates One For All: Full Cowl and Eijiro Kirishima activates his Hardening. Using their enhanced might, Izuku and Eijiro shatter the wall together.
Tamaki surmises that Chisaki is stalling them for time to hide everything and escape and starts becoming stressed out since their lives could be at stake. Mirio encourages Tamaki by saying that they will definitely rescue Eri. Mirio decides to go on ahead and phases through. Tamaki starts blaming himself for falling under pressure.
Mimic causes the floor to open up beneath the Heroes, causing the heroes to fall down a single floor. The heroes are in a hall; three figures appear from the dust, revealing themselves to be three members of the Eight Expendables. As Fat Gum prepares for battle, Tamaki decides to battle the three Eight Expendables himself in order to make up for falling under pressure.
Suddenly, Tamaki, while manifested into a clam, appears before the group of Eight Expendables. He opens up the clamshell and grabs hold of the three Eight Expendables with his octopus tentacles. he tells the three Eight Expendables that he will be their opponents before throwing them onto the ground. Tamaki manages also to grab their guns and swords and crushes them by manifesting a crab claw. Tamaki sees that the three Eight Expendables are buying time so that the ringleaders can escape with Eri and asks the Heroes and Police Force to go on ahead. Eijiro tries to stop him, but Tamaki is resolute in stopping the three Eight Expendables on his own since Heroes like Shota are needed on ahead and tells Fat Gum that he will be fine. Before leaving, Shota disables the three Eight Expendables' Quirks. Tamaki pins the three down with his octopus tentacles and asks them to help Mirio since he tends to push himself too hard.
Tamaki decides to put them to sleep and strikes them with his octopus tentacles. However, Tamaki injures himself after striking Setsuno's mask who reveals that he was hiding a knife in his mask. Shota's Erasure has ended, allowing Hojo to activate his Crystallization and to grow crystals from his body which allows him to break free from Tamaki's octopus tentacles. Tamaki manifests his hand into a clam, but suddenly, it disappears because Setsuno used his Quirk to steal it from him. Setsuno remarks on their status as being the Eight Expendables; they had given up on life but their boss, Kai Chisaki, picked pieces of trash like them from the garbage heap and put them to use again. Hojo is prideful in being just trash and is more than obliged to fulfill the expectations of Chisaki. Hojo strikes Tamaki with his crystallized punch.
Tamaki manages to block Hojo's crystallized punch by manifesting strong crab claw fingers and grabs hold of Hojo's fist with a strong grip. Fueled by Mirio's confidence in him, Tamaki tells the three Eight Expendables to give up because he is Suneater, which is his Hero name that was inspired after Mirio told him that he would outshine the sun.
However, despite Hojo, Setsuno, and Tabe being those worthless people, Kai Chisaki welcomed them with open arms into the Eight Precepts of Death, stating that they should not be rotting away. This is why Setsuno does not care about being captured or any other consequences because Chisaki gave them worth and vows to kill anyone who gets in his way. Tamaki sees that Hojo, Setsuno, and Tabe have been brainwashed into thinking that they have worth and asks the three Eight Expendables if they are fine with being used and thrown away.
Hojo ignores Tamaki's question as a Hero like him could never understand and decides to finish Tamaki off by striking him with his crystallized arm. Tamaki manifests a chicken leg and uses its talons to flick a crystal shard at Setsuno, which distracts Setsuno and causes him to recoil in pain, preventing him from using his Larceny since he can only steal objects that he sees. Hojo strikes Tamaki with his crystallized arm. However, Hojo is shocked that his crystallized strike has been mitigated. Suddenly, Hojo's crystallized body has been clutched by Tamaki's large crystallized chicken leg. Tamaki is impressed with the trust the three Eight Expendables have in each other which has made them a good combination. As Tabe goes on the offensive while Setsuno is still recovering, Tamaki comments that while he may not understand their circumstances and anger, he does understand the hardened bonds they speak of.
Tamaki extends his large crystallized chicken leg, smashes the crystallized Hojo into Setsuna and Tabe, and then smashes the three into the wall, further increasing the damage. Hojo, Setsuno, and Tabe have been defeated. Tamaki states that the hardened bonds they spoke of the show that they are not just using each other for their own gain and comments that friendship is a powerful thing.