Adrian Gecko, known in the Japanese version as Amon Garam, is a character introduced in the third year of the anime series. He arrives at Duel Academy as a transfer student from East Academy, quickly establishing himself as a top-tier duelist. His background is marked by a deep sense of abandonment, as he was left by his birth parents when he was young, which led him to feel he had nothing to live for. He was later adopted by the powerful Gecko Family, a conglomerate with vast influence over global industries like petroleum, heavy chemicals, and information technology. Initially intended to be the heir, his position became complicated when the family later had a biological son, Shido. This situation created feelings of jealousy within Adrian, though he ultimately chose to work diligently for his younger brother's sake rather than act on his darker impulses.

Adrian Gecko is portrayed as a reserved and secretive individual, though his personality differs notably between the original Japanese version and the English dub. In the original, he possesses a flexible intellect and a highly philosophical approach to dueling, once quoting René Descartes Cogito ergo sum to express the meaninglessness of passionate battles. In the English adaptation, he is presented as more of a witty and somewhat laid-back party animal, occasionally breaking the fourth wall with self-aware humor. Regardless of the version, he remains extremely cool under pressure and is technically savvy, always carrying a digi-recorder to take meticulous notes on his observations.

Adrian’s motivations are complex and shift as the story progresses. Initially, his actions appear to serve the interests of his adoptive family and himself, working undercover to investigate the plots at Duel Academy. He survives a dangerous Survival Duel against Chazz Princeton, designed by Professor Viper, but succumbs to the darkness in his heart when he finds the machine causing the energy absorption and chooses not to shut it down. His role evolves dramatically when the group is transported to an alternate dimension. Feeling slighted that the powerful spirit Yubel chose another host, Adrian becomes determined to prove his worth. He later returns in a third dimension with Yubel's power imbued in his arm, intending to become the king of that world. His stated goal is to eliminate poverty and suffering, creating a better world, though Yubel mocks this as a hollow ambition. Tragically, his quest for power leads him to sacrifice Echo, the only woman he ever loved, to free the spirit of Exodia the Forbidden One. Despite his cold actions, it is revealed that he never stopped loving her, and her loss becomes the source of his ultimate regret.

Throughout the series, Adrian’s key relationships define his trajectory. His dynamic with Chazz Princeton is one of mutual recognition, as both share harsh pasts at their respective academies, though Adrian ultimately dismisses Chazz as nothing special. His relationship with the spirit Yubel is antagonistic, driven by his desire for the power she denied him. However, his most significant bond is with Echo, his assistant and lover. Her death at his own hands to fuel his ambition is the turning point that humanizes him, and he carries the pain of that loss with him until his own demise. He also interacts with Jaden Yuki, acting as a secondary antagonist who ultimately falls to the primary threat.

Adrian undergoes significant development, transitioning from a calculating, neutral observer to a tragic villain. His initial jealousy and inferiority complex transform into a god complex in the alternate dimension. His defeat by Yubel, who uses Echo’s lingering darkness against him, is poetic. As he loses the final duel, he cries out in apology to Echo, demonstrating that his love for her was genuine and that his quest for kingship left him with nothing but remorse. After Jaden defeats Yubel, it is revealed that those who died in the dimension were merely trapped, but Adrian is one of the few who does not return, leaving his final fate ambiguous.

As a duelist, Adrian is notable for using two distinct decks. He initially plays a Cloudian deck, which utilizes Fog Counters to destroy monsters and inflict effect damage. His signature card, Cloudian Eye of the Typhoon, can shift the battle positions of all opponent's monsters when it attacks. Later, after obtaining power from Yubel, he switches to an Exodia deck. This deck focuses on assembling the five pieces of the Forbidden One for an automatic victory, utilizing cards like Exodius the Ultimate Forbidden Lord to recycle pieces from the Graveyard. In the manga adaptation, he plays a Forbidden Beast deck, which features monsters that resemble the Exodia pieces and require specific spells to unlock their full potential.