Garou is a former disciple of Bang, also known as Silver Fang, who rejects his master's path to become a self-proclaimed "human monster" and the "Hero Hunter". He is a central antagonist whose actions drive much of the conflict, challenging both the Hero Association and the Monster Association.

As a child, Garou was an outcast who experienced severe bullying. During a game of "heroes," the popular child forced Garou to play the role of a monster and used the game as an excuse to torment him. When Garou fought back, the other children and even the teacher blamed him, enforcing the idea that he was the villain simply because the majority sided with his bully. Watching heroes on television always triumph over monsters further cemented his view that the world was fundamentally unfair. He identified with the monsters who always lost despite their efforts, leading him to vow that he would one day become an unbeatable monster who could overturn this unjust system.

Driven by this childhood trauma, Garou sought out Bang's dojo to master martial arts. He quickly became the star pupil, proving to be a prodigy capable of breaking all but one of a stack of ceramic tiles that his seniors could not manage. However, believing he had nothing left to learn, he rampaged through the dojo, defeating all his fellow students before being beaten and expelled by Bang himself. Before fleeing, he stole the secret technique scrolls, learning the Exploding Heart Release Fist.

Garou's personality is a complex mix of arrogance, fierce determination, and a deeply buried sense of justice. He is rebellious and prideful, viewing most heroes as hypocrites who are merely playing a role for fame and approval. He disguises himself to enter martial arts tournaments and studies the Hero Association's almanac to learn his targets' strengths and weaknesses. Despite his stated goal of becoming "absolute evil," he consistently demonstrates that he is not truly malevolent. He refuses to kill heroes when he has the chance, protects a young boy named Tareo who reminds him of his own childhood, and is frustrated when his actions inadvertently save people. True monsters recognize the doubt and humanity within him, noting that he is not truly evil but is simply confused and lashing out. His ultimate aim is paradoxical: by becoming a symbol of terror that defeats all heroes, he hopes to unite humanity against him, thereby ending bullying and injustice forever.

His key relationships define his journey. His former master, Bang, still cares for him and seeks to stop his rampage, while Garou respects Bang's strength but resents his authority. He shares a rivalry with Metal Bat, whose fighting spirit he comes to respect, and sees the powerful but insecure Superalloy Darkshine as a symbol of hypocritical "hero play". His relationship with Saitama is pivotal. Garou repeatedly fails to recognize Saitama, who casually defeats him on multiple occasions. When they finally clash seriously, Saitama refuses to see him as a monster, recognizing his actions as those of a troubled person with heroic instincts, which forces Garou to confront the contradiction within himself.

Throughout the story, Garou undergoes significant development, starting as a simple hero hunter who targets low-ranking heroes and rapidly escalating to defeating multiple S-Class heroes. He pushes himself to his absolute limit, fighting while grievously wounded and constantly adapting to his opponents' techniques. His body begins to undergo "monsterization," transforming him into a more powerful and fearsome being as he abandons his humanity in pursuit of his goal. This transformation culminates in an awakening that allows him to fight several of the world's strongest heroes simultaneously. However, his development reaches its climax when he loses to Saitama, who helps him realize the error of his ways and see that his true desire was to be a hero, not a monster.

Garou's notable abilities are rooted in his mastery of martial arts, specifically Bang's Water Stream Rock Smashing Fist, which is designed for fighting humans and allows him to deflect and redirect attacks. He possesses superhuman speed, strength, endurance, and reflexes, allowing him to dodge attacks from powerful heroes and withstand immense punishment. His most frightening trait is his ability to adapt and learn during combat; he can analyze an opponent's fighting style and counter it almost instantly, growing stronger from every battle. As he nears his limit, his body undergoes monstrous transformations that grant him even greater power, horns, and a demonic appearance, pushing his threat level to well above standard Dragon-level monsters.