Gigantomachia is a towering and immensely powerful villain who serves as one of the most loyal and devastating assets to the series primary antagonist, All For One. His name is a reference to the Gigantomachy, the legendary war between the Olympian gods and the Giants in Greek mythology, which foreshadows his role as a force of nature capable of challenging the very foundations of hero society.

Standing as an enormous, muscular giant with jagged, rock-like protrusions on his jaw and back, Gigantomachia possesses a bestial and mountainous silhouette. His brown hair is spiky and wild, and his teeth are prominently pointed, with lower canines that jut outward from his mouth. Perhaps his most distinctive accessory is an old-fashioned analog radio that he always carries, usually tied to a rope and worn around his neck. This device is his primary link to his master, serving as a conduit for orders and a tool for emotional regulation.

The foundation of Gigantomachia's personality is an absolute, fanatical devotion to All For One, whom he refers to as his lord and master. He speaks in a manner that can seem religious or reverent, believing everything he does is for the sake of his master. This loyalty is so profound that the mere sound of All For One's voice, even from a recording played on his radio, can instantly calm him from a destructive, blind rage, causing him to become docile and even nuzzle the device. Outside of this conditioning, his demeanor is hot-tempered and impatient, particularly when he is frustrated or when people are uncooperative. He has a simple and somewhat childlike dependency on authority, which has led other characters to describe him as the ultimate order-waiting human. He holds a strong, almost mythic belief in what constitutes a king: someone who inspires dread, is admired, and is undeniably strong.

Gigantomachia's central motivation is to serve his master, All For One, and to see his will carried out. After All For One's defeat and imprisonment, his purpose becomes intertwined with that of Tomura Shigaraki, whom All For One designated as his successor. However, Gigantomachia does not accept Shigaraki immediately. He views Shigaraki as weak and unworthy, and his disappointment is so severe that he breaks down in tears at the perceived inadequacy of the new generation of villains. He exists as a trial for Shigaraki, a massive obstacle that the fledgling leader must overcome to prove he is a worthy heir to All For One's power and ambition.

In the story, Gigantomachia initially remains a hidden, mysterious force, sequestered in the mountains on All For One's orders. He is unleashed more fully when the League of Villains seeks to bolster their power. Upon finding them, he attacks the entire League repeatedly over the course of a month and a half, testing their strength and resolve. His role becomes critical during the conflict with the Meta Liberation Army, where he single-handedly tears through their forces to reach Shigaraki. It is only upon witnessing Shigaraki unlock his true potential and achieve a decisive victory that Gigantomachia finally acknowledges him as his new master, shedding a tear of joy. In the ensuing Paranormal Liberation War, he serves as a living battering ram, carrying the League into battle and plowing through multiple cities and countless Pro Heroes, causing catastrophic levels of destruction and loss of life.

His most significant relationship is, of course, with All For One, his creator and the object of his lifelong devotion. His relationship with Tomura Shigaraki evolves from outright rejection and hostility to fervent acceptance and service after witnessing Shigaraki's power. He also interacts with Dr. Kyudai Garaki, the scientist who helps manage him and uses recordings of All For One's voice to control him. Despite his immense loyalty, his devotion is not without limits. When he comes to believe that he has been abandoned by All For One, he feels a deep sense of betrayal and sadness, a development that later influences his actions.

Gigantomachia is one of the rare individuals who can naturally handle multiple Quirks without becoming a mindless Nomu, possessing a total of seven distinct abilities that make him virtually unstoppable. His original Quirk is Endurance, which converts his morale and willpower into immense physical stamina and energy, allowing him to fight for days without rest. To supplement this foundation, All For One granted him six additional Quirks. Pain Blocker renders him completely unable to feel pain. Gigantification allows him to exponentially increase his already colossal size, growing up to an estimated 25 meters in height as he becomes more agitated or excited in battle. Dog provides him with highly acute senses of smell and hearing, allowing him to track targets over vast distances and even detect enemies while sleeping. Energy Saver enables his body to function on minimal food, water, and sleep. Fierce Gains hardens his muscles to exceptional, near-indestructible levels. Finally, Mole grants him the ability to grow massive claws and back spines, transforming his lower jaw into a protective visor so he can burrow through the earth at high speeds, which All For One intended for stealth and travel.

The price for this overwhelming power is a notable reduction in his cognitive abilities and independent thought. He struggles with complex strategies and is heavily reliant on direct orders, which reinforces his beast-like, instinct-driven behavior in the absence of guidance from his master. Nevertheless, his physical capabilities are so immense that he has forced veteran heroes like Gran Torino to retreat, withstood the flames of Dabi without significant injury, and tossed the giant hero Mt. Lady aside as if she were weightless. He is a walking calamity and arguably one of the most physically destructive forces in the entire series, whose rampage left thousands dead and Japan in a state of complete disarray.