Gemini Defteros, also known as the "Demon of Kanon Island" and addressed by Asmita's ghost as "a man who can destroy the stars of a galaxy", first reveals himself as a man of incredible savagery, although he is at heart a serene warrior. He lives by the philosophy of survival of the strongest. Twin brother of the late Gemini Aspros. Defteros' name, alternatively spelled as Jemini no Defuterosu, meaning "Second one", reveals his condition as the younger of the twins. During his younger years, Defteros suffered the rejection of Sanctuary as a Saint apprentice. Forced to wear an iron mask to hide his existence from the outer world, Defteros was protected by his older brother Aspros, who was considered the rightful future heir of the Gemini Gold Cloth.
In Kanon Island, Defteros helps Tenma to reach briefly the 7th Sense and to understand how to combine the mastery of Cosmo in order to become one with nature as well. After training the young Saint, Defteros leaves telling an unconscious Tenma to take good care of the Pegasus Cloth, possibly out of respect for Asmita whose blood restored it.
Later on, he is seen healing Libra Dohko's wound caused by Hades' sword by using Athena's blood. He then asks him to carry a vial of the same blood to Athena's statue in Sanctuary, and reveals to him his true identity as the Gemini Gold Saint. He reveals to Dohko that his life was saved by Bennu Kagaho, by bringing him to Kanon Island.
He enters the war against Hades in Sanctuary, surprising Bennu Kagaho to save Dohko's life by means of opening an alternate dimension with his Another Dimension attack. He tells Kagaho to convey a message to Hades, that they're arriving to the Lost Canvas and that his defeat is near. He later removes the secret of Athena's Cloth from Dohko's memory and sends him with it to the Lost Canvas.
Remaining within the vacuum of the Another Dimension, Defteros is challenged by his brother Aspros, who was revived by Hades in exchange of his loyalty and the secret of Athena's Cloth. Later, they resume their encounter in the 4th Demon Temple: Mars, which now resembles the Temple of the Twins in Sanctuary. Both warriors engage each other in battle while reminiscing about their past.
It was revealed that Defteros was manipulated in order to assassinate the Pope by his older brother, Aspros, with the attack Demon Emperor's Delusional Fist, a technique that grants the user complete control the target until the victim has witness the death of someone. Defteros was able to fight off the effects of the technique with help of Virgo Asmita long enough to deliver a fatal blow to his brother, completely freeing himself. After his defeat, Aspros applies the Delusional Fist on himself before dying. Defteros took Aspros' body with him and the Gemini Gold Cloth to Kanon Island.
Returning to the battle at the Mars Temple, Defteros is put under the control of Aspros again. To counter the attack, Defteros exhibits a newly introduced technique, the Mavros Eruption Clast, (from the Ancient Greek μαύρος, "black") with which Defteros summons magma from the depths of earth to create a violent explosion. Defteros reveals to Aspros that he surpassed his own limits during his training at Kanon Island and uses his technique to free himself again from his brother's control. Even so, Aspros counters the technique easily with his Galaxian Explosion, overpowering the Gemini Saint into submission. Putting aside fond memories and respect for the once-gentle brother he knew, Defteros engages Aspros in a final effort, pitting his own Galaxian Explosion against his brother's, out of which only one will come as survivor and final victor. Seizing the moment of the clash of the Explosions, Defteros hits his brother with a technique that sacrifices his own life, nullifying the effects of the Delusional Fist on Aspros, making him return to his senses. Afterward, Defteros' body vaporizes, leaving behind the Gemini Gold Cloth. Having sacrificed his life in order to restore his brother's sanity, Defteros remained content in merging his soul with his brother's.
Gemini Aspros is Gemini Defteros' late twin older brother. Once a virtuous Saint, Aspros was executed for plotting against the Sanctuary and wanting to steal Athena's Cloth to rule over Sanctuary. When the brothers reminisce about their past, it is revealed that rumor in Sanctuary had Aspros as the virtual successor of the Pope, however, the latter chose another prospect he considered the fittest. Enraged for having failed in attaining his goal of becoming Pope of the Sanctuary, Aspros violates the sanctity of Star Hill, a place reserved only to the Popes, and steals secrets kept there for ages. It is also revealed that the year his execution took place, Aspros manipulated Defteros with the forbidden technique that he learned in Star Hill, the Demon Emperor's Delusional Fist, in order to assassinate the Pope of Sanctuary, taking advantage of the fact that Defteros existence was almost totally unknown. As Defteros trespassed the Chambers of the Pope to fulfill his mission, he was stopped by his brother, who pretended to protect the Pope and to be ready to kill his brother in punishment for treason. As the Pope turned his back to avoid the scene, Aspros prepared to kill him, only to be stopped by Virgo Asmita. After Defteros was freed by Asmita from the effects of the Delusional Fist, Aspros received his brother's lethal attack.
The Pope reveals that his seat was destined to be Aspros' had he not deviated from the path of virtue. Humiliated by his defeat at the hands of whom he considered a shadow, Aspros applies his Delusional Fist on himself, to assure he would achieve his goal even if having to return from beyond the grave. His body was taken by Defteros to Kanon Island. After being resurrected by Hades, he appears before his brother immediately after the latter saves Libra Dohko from Bennu Kagaho, and engages him in battle. Revealed by Hades to be also the guardian of the Fourth Demon Temple: Mars. As a servant of Hades, he was granted the Gemini Surplice, a dark reflection of the prideful Gold Cloth he once wore. In the past, 16 years before the events depicted in Lost Canvas, Aspros shared with Defteros the dire trials required to become a Saint. Aspros was very protective of his younger brother, who was considered a wasted life born under an unfortunate star. Years later, bearing a grudge against his brother for his execution, Aspros tries unsuccessfully to kill his twin during their encounter in the Mars Temple, with their signature and most destructive technique: Galaxian Explosion, a massive burst of Cosmo that easily vaporizes opponents, said to be capable of smashing stars.
Aspros overpowers his brother with ease, and reveals his gentle nature of the past was but a facade, as his goal of domination required him to become purely evil. Determined to end the battle, Aspros clashes with Defteros in a final effort. Consuming his brother's Galaxian Explosion with his own, Aspros believes himself the victor, only to realize that during the clash, his brother sacrificed his life in front of him, negating the effect of the Delusional Fist, and thus making him return to his old, good-natured self, and is accepted by the Gemini Gold Cloth. Aspros then turns on Hades revealing his intentions, only to be stopped by Mephistopheles Yōma, who dissolves his body into quantum particles.
After Yōma later dissolves Shion with his Marvelous Room technique, Aspros reappears carrying with him Dohko and Shion, to engage Yōma in battle. In order to prevent the Specter from stopping time, Aspros transport their fight into the Another Dimension, a place where time is always flowing, moving at lightspeed. Then Aspros survived Yōma's Rewind Bio due to his undying nature as a Specter and forced Yōma to reveal his true form with Galaxian Explosion. Aware that his opponent is a god, Aspros prepares to unleash his full power. After a fierce battle, Aspros manages to open a dimensional gateway to the Heavenly Realm, which destroys Yōma's body completely, and seals his soul away within the 108 beads mala forever. Aspros' temporary life as a Specter reaches its end, and he states his repentance for his crimes to Athena, delivering the 108 beads mala to her. Having fulfilled his duty, Aspros' body dissolves, leaving the Gemini Surplice and the Gemini Gold Cloth behind, and his soul departs to eternity.
He last appears in spiritual form during the attack of golden light unleashed by the Gold Saints on Hades.
