After Saga was consumed by his evil personality, he murdered the Pope and started impersonating him to control the Sanctuary and further his ambitions. To hide his true identity, he wears a helmet that obscures his face at all times.
To gain power, he conspires with Chronos, Pontos, and the Titans, With Chronos giving him a golden dagger that has the power to kill gods. Saga uses it to attempt to kill the infant Athena, but Sagittarius Aiolos interrupts him and saves her, also discovering his identity. As a result, Saga declares Aiolos a traitor of the Sanctuary, framing him of his assassination attempt.
He is a morally complex character. As in Saint Seiya, the two sides of his personality often argue. After Chronos arrives at Sanctuary to reclaim the Megas Drepanon, his evil side orders the Gold Saints not to engage in battle. His good side, however, is unable to stand by and rises to fight the Titan. Saga uses the golden dagger Chronos had once given him, but before he can kill Chronos, his evil side takes control again and returns to the Pope's throne, where he later gives the order to all Saints prepare for battle.
13 Years Conspiracy
Along with fellow senior Sagittarius Aiolos, Saga was summoned by Pope Shion for a meeting to decide the successor to the throne. Due to the fact that most of the Gold Saints were too young at the time for this honor, the choice fell upon Gemini Saga and Sagittarius Aiolos. Unfortunately for Saga, Aiolos would be the one announced to take the seat and lead infant Athena towards the path of good. This news did not please the Gemini Saint, and it brought him to an eventual confrontation with Shion.
Gazing at the constellations on Star Hill, Pope Shion recalled the words of prophecy said by the last Pope before him. Suddenly, Saga appeared and demanded to know why he, the superior Gold Saint, had been denied the throne. Shion hesitated at first, but finally explained that he felt an uncomfortable presence residing within Saga. Saga's hair color suddenly faded from light to dark, and his eyes turned blood-red with black pupils -- the transformation confirming Shion's suspicions. Before the Pope could react, however, Saga landed a fatal blow through his heart.
With Shion's death, Saga assumed his identity and orchestrated a mass conspiracy that would last for 13 years. The Pope's head-dress and mask offered Saga room to hide his true face from the rest of the world, and soon, rumors began to circulate throughout Sanctuary that the Gemini Gold Saint had vanished. Saga, using the Pope's disguise, then attempted to assassinate the baby Athena -- an event that would result in the banishment and death of Aiolos. Pulling baby Athena away from the tips of Ares' mysterious golden dagger, Aiolos demanded an explanation. Ares ignored the saint and resumed his attack, which led to Aiolos retaliating with a blow of his own. The punch sent Ares flying onto the nearest wall, knocking off his head-dress and unmasking Saga in the process. The revelation shocked Aiolos, for he now realized the true depths of this heinous confrontation. With his cover blown, Saga unleashed a fury of assaults that heavily injured Aiolos. Aiolos, concerned for the infant Athena's safety, quickly took off from Sanctuary with the child in his arms.
Unfortunately, the rest of Sanctuary recognized this as an act of treason. Saga, content in knowing that his true identity was still hidden from the rest of the world, declared Aiolos a villainous traitor who had ran off with the baby Athena. Aiolos himself managed to escape, but on his way to the outside world, he encountered the man responsible for his fatal injuries -- the Capricorn Gold Saint. Aiolos would not survive to see the next day, and with the only person who knew the truth dead and the infant Athena lost, Saga truly had free reign to rule Sanctuary as he pleased.
Supreme Pontiff Ares
Shortly after the start of the series, Saga had apparently became tired of hiding under the guise of Shion. He spread word that "Shion" had finally passed on, and that the seat of the Pope would be "passed down" to Shion's younger brother. The "new" head in charge of Sanctuary called himself the Supreme Pontiff Ares, and his reign transformed Sanctuary into a much harsher environment. Of course, the identity switch was a ploy, and it was still Saga underneath the newer head-dress and cloth.
Evidently, Shion's younger brother must not have been an influential or even popular figure, as his name allowed Saga the freedom to make decisions without instigating too many suspicions about his acts, or developing any uprising that may compromise his goals to achieve total domination. Ares was projected as a cruel dictator, and often acted in various ways that assured his dominance over Sanctuary's saints. Even Sanctuary itself slowly evolved to a more malevolent structure, with trainers delightfully enforcing harsher training methods on their students, as well as saints having no qualms about killing each other to demonstrate their strength.
After Saori Kido had declared to the world that she was Athena, and gathered several bronze saints to support her claim, most in Sanctuary instantly felt enraged that some outsider had dared to impersonate their patron goddess for her own personal gain. Ares took advantage of the fury his saints felt and deployed them to punish this young lady and anyone who aligned themselves with her. Of course, deep down, Ares knew that Saori Kido was the true reincarnation of the goddess that he had failed to assassinate 13 years ago. He also felt distraught that she had taken custody of the Gold Cloth of Sagittarius, a prestigious object with the potential to tide things towards her favor should she ever find a suitable replacement for Aiolos. Thus, Ares quickly commanded his army of saints, which included both Silver and Gold ranks, to annihilate Saori Kido and her band of warriors.
As time went by, many in Sanctuary began to question Ares and his decisions. Many were baffled that almost none of the silver saints sent to Japan returned, and they were more shocked to find that even gold saints had apparently failed in their mission to capture or kill Saori Kido and her weakling bronze saints. Speculations and reports slowly spread that, perhaps, Saori Kido was not bluffing when she claimed that she was Athena...?
It became more and more apparent that the evil nature in Saga hadn't fully consumed him, and somewhere deep down lie the true magnanimous personality. The series depicted various scenes of the two sides arguing back and forth with each other, and the clash finally accumulated during the final fight between Saga and the Bronze Saints Pegasus Seiya and Phoenix Ikki. The benevolent traits of Saga awoke on several occasions to disrupt his evil side, each time with the goal to intercept his evil nature's would-be-fatal assaults on Seiya. Even with the aid of Saga's true personality, the fight was a gruesome struggle from beginning to end. In the manga, it was Seiya (using the shield of Athena) who at last ejected the evil spirit from Saga, and in the anime, it was the good nature in Saga himself who finished off the evil entity. As the arc came to a close, Saga was finally cleansed once and for all, and, in a final attempt to restore his honor, committed suicide.