All Might's debut as a Hero deeply inspired Chizome. Since then, Chizome aimed to become a Hero like All Might. He entered a private Hero course high school to train to become a Hero, however, he became hopeless and despaired after seeing the school's "fundamental decay", as the students there were becoming Heroes for the money and personal gain. Disillusioned by the reality of Heroics, Chizome dropped out during the summer after his first year.
After dropping out of high school, the teenage Chizome preached an ideology which he called the "Revival of Heroics" through street oratory speeches and soapbox speeches, but his words went unheard.
Over the following decade, Chizome realized that only action could create his ideal world and change Heroics for the better. Chizome researched and disciplined himself in the training of the killing arts through self-study in order to achieve his duty of "Revival of Heroics". During this decade, Chizome's parents died. It is unknown what led to their deaths but it is believed that Chizome was not involved.
As the vigilante Stendhal, Chizome hunted villains to make up for the heroes' mistakes. During this time he worked with the vigilantes such as Knuckleduster, The Crawler, and Pop☆Step. For some time, Stendhal became more and more disillusioned with hero society to the point where he began to work with a Kuin Hachisuka, who was supplying the Quirk-enhancing drug Trigger to several small-time criminals. He brutally murdered Akira Iwako and several Yakuza members before going after Soga Kugizaki. In a single battle with Knuckleduster, Stendhal's nose was completely smashed in and he was forced to retreat, causing minor injuries on the Vigilante. However, his defeat was not a complete loss, as became enlightened after hearing Knuckleduster's comments on resolve. After retreating, Chizome betrayed Kuin by trying to stab out her eye and soon after, he cut off his broken nose.
Sometime later, Chizome took up the mantle of Hero Killer: Stain and begun advocating the "Revival of Heroics". In this ideology of "Revival of Heroics", Stain stated the guidelines of being a hero: that heroes must not desire rewards or compensation, and that people can only have the title of hero when they commit the ultimate acts of self-sacrifice. The ideology stated that the "heroes" of the modern world are fakes who pretend to be heroes.
Stain then began implementing his "Revival of Heroics" by killing pro heroes, deciding that he would not stop his purge of the pro heroes until the world realized the flaw in the Heroics system. Stain's end goal of implementing his ideology "Revival of Heroics" was to create a just world filled with true heroes.
After having already killed seventeen heroes and injured twenty-three beyond recovery, Stain finds his way to Hosu, where he is confronted by Ingenium. Stain heavily injures the pro hero and leaves him for dead after monologuing about his disgust over those he believes are not worthy of being called heroes. He believes the only person who is allowed to do him in is All Might, and that only he is a true hero.
Stain moves to a rooftop to scout potential targets, but he is suddenly confronted by Kurogiri. He threatens the villain with his katana and Kurogiri asks him to hold his blade. He proposes a meeting between the League of Villains and the notorious Hero Killer: Stain and then teleports them away.
Kurogiri brings Stain to the League's hideout to meet with Tomura Shigaraki. He insists that Tomura allow Stain to join their party. Stain questions the goals of the villains and Tomura replies that he desires to kill All Might and anything he does not like, including a particular U.A. student. Unimpressed by Tomura's childish nature, Stain threatens them both because he believes Tomura is the worst kind of person and that there is no sense in bloodshed without a cause.
Stain immobilizes Kurogiri and pins down Tomura while cutting into his shoulder with a knife and holding another to his neck. He preaches to his overpowered foe that he will remain a weakling who will achieve nothing with real convictions or desires. He goes on to state that the world is overrun by fake heroes and villains who flaunt their power and chase petty dreams and that they must all be purged.
As Stain's knife draws closer to the Tomura's face, the villain reacts by quickly grabbing the knife and disintegrating it with his Quirk. Stain retreats a short distance and Tomura responds to his ideology. Tomura reveals that he is driven by the desire to kill All Might and destroy the people who worship him. Although Stain thinks that their goals oppose one another, he agrees that they both want to destroy the present. Tomura demands Stain leaves, but Stain reveals that he threatened him to test his motives because people show their true selves while on the verge of death. He says he will allow Tomura to grow and demands to be returned to Hosu.
Kurogiri warps himself, Stain and Tomura to Hosu. Tomura asks what Stain plans to do there, to which he replies he will reform Hosu. He plans to do so by killing the false heroes whose only desires are money when heroes should be those who achieve great deeds. He leaps off the rooftop, stating that he will continue to appear until society realizes its error. In the seven places that he has appeared so far, Stain has attacked at least four pro heroes. He has only attacked Ingenium in Hosu and decides to attack the pro hero Native next. After paralyzing Native, Stain notices the chaos ensuing in the city and connects it with the League of Villains. He says he will deal with them after Native, but is then interrupted by Tenya Iida. Tenya tries to attack Stain, but he notices the young man first and knocks him down. He demands the child leave, but Tenya declares that he will stop the Hero Killer in the place of Ingenium. Due to Tenya's selfish and vengeful desires, Stain decides to take it upon himself to purge him from the world.
Tenya attacks with a high-speed kick, but the Hero Killer evades and pierces Tenya's arm with his spiked boot. Then he kicks Tenya to the ground and says both Ingenium and his brother are weaklings. He reveals he allowed Ingenium to live to spread the word of the Hero Killer's exploits. Stain claims that neither of them has the right to be called a hero. Tenya refutes this and threatens to kill Stain for his brother. Stain responds by telling Tenya that he should be focused on helping Native and that his lust for vengeance is blinding him and becoming the furthest thing from what a hero should be. Just as the Hero Killer prepares to claim another victim and sacrifice Tenya for a more just world, Izuku Midoriya leaps in out of nowhere and punches Stain back.
Izuku found Stain by going by what he had heard on the news. He knew that Stain attacked all his victims in low populated areas and searched all the back alleys by Manual's office until he found them. Stain commends Izuku for saving his friend but warns him that if they clash the weaker of them will be culled.
Izuku decides to stand up to Stain as a hero, impressing the Hero Killer with his strong conviction. Their bout starts and Stain is further roused when Izuku closes the distance between them to negate his opponent's longer reach. Stain strikes Izuku with another blade but the young man narrowly evades by moving through Stain's legs. Stain adjusts by slashing behind him, but Izuku swiftly moves into the air above him and strikes Stain with a 5% Detroit Smash attack.
The Hero Killer is barely rocked by Izuku's smash and manages to lick a trace of Izuku's blood from his knife. Stain claims he saw through Izuku's plan, but praises the young man and says he is worth allowing to live. He decides to try and kill Tenya instead but is once again interfered with when Shoto Todoroki arrives to save his friends. Shoto Todoroki nearly blasts Stain with a stream of fire, forcing him to dodge. He continues to attack with ice and fire to continue forcing the Hero Killer away and declares that the villain won't kill anyone tonight. Stain grazes Shoto's cheek with a throwing knife and rushes him while uses his sword as a distraction to get in close and lick the blood from his face. Shoto uses his fire to keep Stain away, garnering the villain's praise, and the pair battle it out. Stain consistently evades Shoto's attacks of fire and ice and manages to pierce his arm by throwing two knives into it. Stain plans to use the opening to kill Native, but Izuku is freed from paralysis and throws the villain away.
Izuku and his allies deduce Stain's Quirk gives him the ability to immobilize people by ingesting their blood, and its time limit is based on blood type. Izuku and Shoto decide to protect Native and Tenya together, and Stain comments that the commencing fight will be difficult. He and Izuku battle in close quarters while Shoto supports him from a distance with his Quirk. Stain eventually cuts Izuku's leg and paralyzes him before attacking Shoto again. Shoto counters by shooting flames at Stain, but he evades and claims that Shoto fights too recklessly because of his powerful Quirk. He nearly cuts the student down, but Tenya comes to the aid of his friend after being freed from Bloodcurdle's effects. He uses his Recipro Burst technique to save Shoto by kicking Stain's sword in half and then repealing the villain with a subsequent kick.
Tenya apologizes to his allies for their injuries, but Stain refutes Tenya's reformation and calls him a fake to society that must be killed off. Stain becomes flustered and becomes more ferocious in his attempts to kill Tenya and Native. Shoto keeps him at bay with his elemental attacks and Stain tries to get him out of the way with a throwing knife. Tenya throws himself in front of it and it pierces his arm. Stain follows up by pinning Tenya down with a second dagger and dives down at the pair of young heroes in training.
He doesn't notice Izuku's time limit is up and the latter rushes him with his power. Tenya does the same, using Recipro Extend to clash with Stain. They both attack the villain at the same time, with a Quirk enhanced punch and kick respectively. Stain makes one last ditch effort to kill Tenya, but he evades and kicks Stain into the air. Shoto uses the opening to burn Stain with a blast of fire. These additional blows cause Stain to fall unconscious and lay on top of a pillar of Shoto's ice. The students take his weapons and tie Stain up to restrain him.
Izuku, Shoto, Tenya, and Native leave the alley with the unconscious Hero in tow. They are met by several pro heroes when a winged Nomu swoops down and grabs the injured Izuku before flying away. The Nomu was injured by Endeavor and its blood gets on the cheek of one of the pros. Stain wakes up and licks it off to paralyze the beast before freeing himself and leaping into the air to stab the Nomu's brain and save Izuku. Despite being heavily injured, Stain kills the Nomu and states that the "fake" Pro Heroes and the pathetic criminals in society are his targets for his purge to create a stronger society.
Stain notices Endeavor's arrival and decides to confront the "false hero" despite being outnumbered. His mask falls off and he shouts his convictions to his opposition. He tells them that the streets must run with the blood of hypocrites, that he will reclaim the word "hero" and goes on to challenge the fakes to stop him. All while slowly stepping towards the group of heroes who are frozen with fear by the Hero Killer's bloodlust. Stain's malefic aura sends chills down their spines as he shouts that only All Might is worthy enough to stop him. He only stops when his lungs are perforated by a broken bone and he loses consciousness. Although unconscious, Stain was the only one present at the scene with enough fight left in him to stand against his enemy.
The next day, the Chief of Hosu Police states that Stain sustained several burns, bone fractures, perforated lungs, and other serious injuries but he is still alive and is undergoing treatment for his grievous injuries. The news of Stain's defeat and arrest spreads like wildfire throughout Japan. Stain's true name and backstory are revealed in published articles and become a major topic. His ideology inspires villains to rally and unite themselves under the League of Villains.
After undergoing treatment for his injuries, Chizome is incarcerated in Tartarus, a special prison for criminals for whom the death penalty is not enough. He watches in silence as All For One is brought in.