Garou is a central antagonist and a self-styled "human monster" who systematically hunts heroes. His actions are rooted in a troubled childhood where he was frequently ostracized and forced to play the role of the villain in children's games, always losing to the popular "heroes". This experience fostered a deep-seated resentment towards the concept of unfair, biased justice, leading him to sympathize with the monsters who were always defeated on television. He resolved to become the strongest monster of all, an embodiment of "absolute evil," believing that such a terrifying, unbiased threat would unite humanity and erase the world's irrationalities, such as bullying and war.

To achieve this goal, Garou trained under the legendary martial arts master and S-Class hero, Bang, quickly becoming his most talented disciple. However, feeling he had learned all he could, he turned on the dojo, brutally defeating all his fellow students before being expelled by Bang himself. For a time, he disappeared, only to re-emerge as the "Hero Hunter," launching a brutal campaign against the Hero Association. He is a highly strategic and adaptive fighter who has studied the Hero Almanac to learn the strengths and weaknesses of his targets. Garou's journey is defined by his relentless pursuit of power, pushing himself to the brink of death in countless battles, which forces him to break his own limits and evolve into a more monstrous and powerful form. He eventually develops his own ultimate technique, the "Killing God Fist of Monster Destruction," a culmination of all the martial arts styles he has absorbed.

Garou's personality is a complex mix of ruthless ambition and a hidden, perhaps even unconscious, sense of justice. He is rebellious, arrogant, and driven, viewing the Hero Association and its ideals as hypocritical. He refuses to join the Monster Association despite their offers, as he finds their ideology of mindless slaughter and chaos just as distasteful as the heroes' hypocrisy. A key relationship in the story is his adversarial bond with his former master, Bang. Bang, feeling a sense of responsibility for creating this threat, relentlessly pursues Garou to stop him, leading to several brutal confrontations. His most significant dynamic is with Saitama, the series' protagonist. In their initial encounters, Saitama effortlessly defeats Garou with a single blow, though Garou fails to recognize him as a real threat due to amnesia or circumstance. Their final confrontation serves as the ultimate test of Garou's philosophy and power, forcing him to confront the error of his worldview. Other notable relationships include his fight with Metal Bat, towards whom he develops a grudging respect, and his protective, if gruff, concern for a young boy named Tareo, who reminds him of his own bullied childhood.

Throughout the story, Garou undergoes significant development, transforming from a simple hero-hunting villain into a truly monstrous figure, and finally, reluctantly, into something closer to an anti-hero. His body and mindset evolve as he survives increasingly difficult fights, eventually approaching the level of a "disaster level god". Yet, his deepest desire is eventually revealed not to destroy humanity, but to save it by becoming the ultimate common enemy. This internal conflict between his monstrous actions and his human heart is the core of his character arc. Garou's notable abilities include his mastery of Bang's "Water Stream Rock Smashing Fist," a martial art specifically designed to counter human opponents. He possesses immense strength, blinding speed, and incredible endurance, allowing him to fight even while severely injured. His most frightening trait, however, is his ability to rapidly learn and adapt during combat, copying and countering his opponents' techniques in real-time, which makes him a more formidable threat than almost any pure monster.