Gaia is a prominent character known as the leader of the First Airborne Brigade from Hokkaido. He is a super-soldier often referred to by the epithet Mr. War. Originally, Gaia was a reckless mercenary named Nomura who loved killing. During the Ugandan Civil War, he was captured and placed before a firing squad, an event that made him face what is described as the ultimate fear. Although he was rescued at the last second, the overwhelming rush of adrenaline and terror caused a split in his personality, leading to the manifestation of the Gaia persona. This traumatic event also caused him to lose all of his hair.

Gaia stands out as a character whose fighting philosophy rejects the idea of building excessive muscle mass. Instead, he believes in caring for the body by giving it rest, freeing it from the control of the mind, and using every part of the surroundings as a weapon. His most distinctive ability is the control of his own adrenaline flow. He can force his body to produce and regulate adrenaline at will, using it to boost his strength, speed, durability, and even his senses to superhuman levels. This ability allows him, despite his small and lean stature, to perform feats such as destroying tanks with his bare hands. He is also highly intelligent, often employing psychological tactics and a well-thought-out strategy in combat rather than relying on brute force alone. His signature techniques include a powerful scream known as the Eardrum-Burster that can disorient opponents, a camouflage technique using sand or dirt to become invisible, and the Vine Art which uses the environment to bind and strangle foes. He also uses projectiles like water, sand, and debris as bullets, and possesses an extremely unorthodox technique called the Tunnel, which involves entering an opponent through the rectum and exiting through their mouth.

In his initial appearance, the personality of Nomura is timid and acts as a medic for his team, while the Gaia personality is a calm, confident, and brutal killer who awakens in moments of extreme danger. However, over time, the Gaia personality eventually absorbs the Nomura one, becoming more balanced and less reckless. He is known to despise Yujiro Hanma, considering him a mere thrill-seeker who fights only for pleasure, and he once declared that Baki is a better warrior than his father. Despite his hatred, he later shows submission to Yujiro after suffering a crushing defeat at his hands.

Gaia serves as a crucial early antagonist and rival to Baki Hanma. Their battle takes place in a forest, where Gaia uses the environment to dominate the fight, teaching Baki a brutal lesson about the difference between an arena match and a true fight for survival. He attempts to use psychological manipulation on the young fighter, claiming that Baki’s own body has turned against him due to his punishing training regimen. Although Gaia nearly kills Baki by hanging him and stopping his pulse, Baki ultimately defeats him by cutting off the blood flow to his brain. After his defeat, Gaia forms a friendly bond with Baki, joining the circle of fighters associated with the underground arena. In later story arcs, he fights alongside Baki and other characters. Notably, he engages the death row convict Sikorsky, defeating him not with raw power but by using psychological warfare and the environment of the arena to break the convicts spirit, demonstrating his continued role as a master of tactical and environmental combat.