Jim Starbuck co-leads the Gemini Defense Squad alongside his sister, Gem Starbuck, within a specialized security force protecting Earth. This brother-sister team operates advanced Gemini Vehicles designed to combat threats.

Severe injuries sustained during an early mission forced emergency bionic augmentation, transforming them into cyborgs. This cybernetic enhancement granted them the unique ability to merge physically and mentally into a single androgynous entity named Aizenborg. This fusion also combines their Gemini Vehicles into a more powerful combat form, but imposes a strict operational limit of 3.5 minutes before separation is required.

As Captain, Jim exhibits a decisive and occasionally rigid leadership style. He prioritizes tactical objectives during missions, leading to conflicts with Gem over operational decisions. This tension surfaces in high-pressure situations, such as when Gem advocated assisting an endangered civilian while Jim insisted on focusing on the primary threat, reflecting his military-influenced chain-of-command perspective.

Jim pilots the primary Gemini Vehicle, serving as the core component during the fusion sequence creating Aizenborg. In this merged form, he contributes to controlling Aizenborg's capabilities, deploying integrated weaponry like wing-mounted sawblades and frontal drills against dinosaur-based adversaries in direct, destructive combat.

Following the defeat of the initial antagonist Ururu (sometimes designated Emperor Tyrannus), Jim and his team confront evolved threats from subsequent villains Gottes and Zobina, who deploy more advanced mutant dinosaurs and mechanical foes. This escalation necessitates their continued reliance on the Aizenborg fusion capability.