Adrian Gecko, known in the Japanese version as Amon Garam, is a new student hailing from East Academy who enters Duel Academy in the third year. Reserved and secretive, Adrian is a neutral character whose actions seem to only serve the Gecko Family and himself. However, after obtaining the spirit of "Exodia the Forbidden One" in the third alternate dimension, he intends to rule it and acts as a secondary antagonist towards Jaden and the primary antagonist, Yubel.
His given name in the Japanese version is derived from an alternate spelling of the name of an Egyptian air deity, Amun.
In the English version, Adrian presents himself as somewhat of a party animal, and is witty to point where he sometimes pokes fun at the series itself, such as in episode 112.
In the original Japanese version, he takes a highly philosophical approach as a duelist, once quoting René Descartes "Cogito ergo sum" to express the meaninglessness of passionate battles such as the Survival Duels announced by Professor Viper.
Adrian is the eldest son of the Gecko Family (Garam Group; ガラム財閥, Garamu Zaibatsu), which has considerable influence over the world's petroleum, heavy chemical industry, and information technology. Abandoned by his parents when he was young, Adrian developed an inferiority complex that convinced him that he had nothing left to live for. He observed the sky and awaited death but was one day adopted by the Gecko Family which intended for him to become its heir in light of the family's failure to produce a biological child to fill the position. However, when a son named Shido was finally conceived, Adrian grew jealous of his younger brother (in the Japanese version, he contemplates killing him), but chose to continue working diligently for his sibling's sake. In the English version, it states that he completely forgot about his vendetta during the timeskip.
From the outset of the third year Adrian works undercover to learn more about Viper's plot and secretly acquire Yubels power for himself, being aware he was affiliated with the spirit. Eventually, he is approached by Professor Viper himself, who threatens to return him to East Academy if he refuses to duel in a Survival Duel. Suspecting that he will set the Bio-Band energy absorption level on maximum—which can cause death—for his scheduled Duel, Adrian invites students to a party to participate in Survival Duels with one another, intending to overload Viper's project. As he leaves the Obelisk Blue dormitory, however, Chazz Princeton lifts him off the ground by helicopter, challenging him to a duel. Intrigued by Chazz's determination stemming from his time at North Academy, Adrian reveals the hellish past that he too has suffered. Despite this similarity, he concludes that Chazz is "nothing special", and that although he wins the duel, the energy absorption level on his Bio-Bands was not minimized enough to prevent him from collapsing.
After recovering from the duel, Adrian infiltrates Viper's hideout. The two spar for ownership of a certain card that Viper possesses after revealing that Adrian also works out at the gym, explaining the result of his built arms, of which the Gecko Group holds interest, but the spirit within the Survival Duel energy chamber restrains Adrian just as he seems to gain the upper hand. Managing to temporarily short circuit the facility’s power source, Adrian escapes and finds the machine responsible for Survival Duel energy absorption, but succumbs to the darkness of his heart and chooses not to shut it down.
While trapped in the alternate dimension along with Jaden Yuki and company, Adrian teams up with them to fight off the duel zombies but mysteriously erases a file about him while on a Gecko submarine, trying to get medicine for an ill Blair Flannigan. On numerous occasions he meets Marcel, who was possessed by Yubel. Asking Yubel why it didn't choose him to use its power Yubel replies that Marcel had an unexpected and greater darkness in his heart at the time. Enraged over this, Adrian continues to monitor Yubel and ends up stranded in the alternate dimension after Yubel is defeated by Jaden Yuki and Jesse Anderson.
Adrian later reappears in the third alternate dimension after the fall of The Supreme King with his left arm imbued with a portion of Yubel's power. Intending to become the dimension's king he sacrificed the only woman he ever loved, Echo, to free the spirit of "Exodia the Forbidden One" from imprisonment and make it his servant, defeating Aster Phoenix in the process. He meets up with Jesse who is now possessed by Yubel in the latter's castle to prove that he is worthy of being king but is defeated, feeling the pain of losing Echo a second time beforehand and dies because losing a duel in that dimension results in death. During that the duel he revealed that he intended to become the dimension's king with intent of ridding the world things such as poverty and suffering, which Yubel mocked him for. Also, it turned out that he never stopped loving Echo and that she still loved him. After losing he says "I'm sorry" to Echo (who is not actually there). He felt guilty of his actions towards her and right before he disappears he screams out Echo's name (in the English version he screams out that he is so sorry for his actions towards her), showing he did truly care for her and loved her.
Adrian is seen one more time prior to Jaden's Duel with Yubel when Jaden remembers all the people Yubel has wronged.
Although it is later revealed that everyone who died in that world was actually trapped in yet another alternate dimension, that Yubel created, Adrian is one of the few who does not return after Jaden's final duel with Yubel; as well as Zane Truesdale, Bastion Misawa, and Jaden himself. Jaden and Zane later return to Duel Academy and Bastion chooses to remain in the alternate dimension with Tania and her friends. Adrian's fate remains unknown.