Adrian Gecko, known in the original Japanese version as Amon Garam, is a character who arrives at Duel Academy during its third year as a transfer student from the prestigious East Academy. He is the eldest son of the wealthy and influential Gecko Family, a conglomerate with vast holdings in the petroleum, heavy chemical, and information technology industries.

Adrian's background is marked by a deep personal tragedy. Abandoned by his birth parents at a young age, he developed a profound inferiority complex and felt he had nothing to live for, once waiting for death while watching the sky. He was subsequently adopted by the Gecko Family, who intended for him to become their heir. However, this changed when the family conceived a biological son, Shido. This event stirred intense jealousy in Adrian; in the original Japanese version, he even contemplates harming his younger brother. Ultimately, he chose to suppress these feelings and dedicated himself to working diligently for his sibling's sake.

Personality-wise, Adrian is portrayed differently depending on the version. In the English adaptation, he often presents a facade of a witty, laid-back party animal, occasionally breaking the fourth wall with self-referential humor. In contrast, the original Japanese version depicts him as a reserved and secretive individual with a highly philosophical mindset, once quoting René Descartes' "Cogito ergo sum" to dismiss the pointlessness of passionate conflicts. Beneath these exteriors lies a calculating and ambitious person whose actions are primarily driven by his own goals and the interests of the Gecko family. He is known for his immense physical strength, a result of his personal training, which is noted during a physical confrontation with Professor Viper.

Motivation is a complex force for Adrian. Initially, his actions seem to serve the Gecko Family and his own survival, as he investigates Professor Viper's sinister plot at Duel Academy. However, his true ambition is revealed when he is transported to an alternate dimension. He seeks to obtain ultimate power for himself, intending to become the king of that world and reshape it into a place free from poverty and suffering. This desire leads him to make a pact with the malevolent spirit Yubel, who grants him a portion of its power, transforming his left arm. His most significant and tragic act is sacrificing Echo, his childhood companion and the only woman he ever loved, to free the spirit of Exodia the Forbidden One and bind it to his service.

Adrian's role in the story evolves from a mysterious and neutral transfer student to a secondary antagonist. He initially works in the shadows, manipulating events and other characters, including Jaden Yuki and his friends, to further his investigation of Viper. After the group is transported to another dimension, he spies on them on behalf of Yubel. His transformation is complete when he chooses to remain in the third alternate dimension after the fall of The Supreme King, defeating Aster Phoenix with his newfound Exodia power and directly challenging a Yubel-possessed Jesse Anderson for control of the dimension.

Key relationships define his arc. His relationship with his adopted brother Shido is a source of inner conflict, as he ultimately chooses to protect Shido despite his jealousy. The most poignant relationship is with Echo, a woman who served as his playmate and companion. He claims to have sacrificed her for power, yet during his final duel, it is clear he never stopped loving her, and his actions are driven by a twisted desire to honor her memory by creating a better world. His relationship with Yubel is one of mutual manipulation; he seeks its power while planning to destroy it, and Yubel eventually exploits the lingering darkness in Echo's heart to defeat him.

Adrian experiences significant development, transitioning from a seemingly loyal family heir to a ruthless power-seeker. His choices lead to his ultimate demise, as losing a duel in the alternate dimension results in death. In his final moments, he expresses profound remorse for his actions, screaming Echo's name in apology before disappearing. Notably, unlike many other characters who perished in the alternate dimension, Adrian is one of the few who does not return to the real world after Yubel is defeated, leaving his final fate unknown.

As a duelist, Adrian is highly skilled and uses two distinct decks. He first employs a Cloudian deck, which focuses on using Fog Counters to control the field, destroy monsters, and inflict effect damage. His signature card from this deck is Cloudian - Eye of the Typhoon, a powerful monster that can shift all opponents' monsters' battle positions when it attacks. After gaining Yubel's power, he switches to an Exodia deck. This deck centers on assembling the five pieces of Exodia the Forbidden One for an automatic victory. His strategies include using Exodius the Ultimate Forbidden Lord, which can win the duel by sending all five pieces to the graveyard, and employing support cards like Break the Seal and Backup Soldier to retrieve the pieces from the deck or graveyard.