Alex first appears in the original Golden Sun, accompanying the game's villains. He subsequently appears in Golden Sun: The Lost Age, and Golden Sun: Dark Dawn, serving a similar role throughout.
He is a Mercury Adept from Imil, thought to be the last of his clan along with his cousin Mia. He is shown to have immense mastery over Psynergy, able to conjure powerful torrents, heal all wounds, and warp at will. Though he makes company with the antagonists of each game, he often provides assistance to the party. Whether this assistance is out of compassion for the heroes or purely to further his own ends (which indeed it tends to) is unclear. Alex is an intelligent man with a polite manner of speech, though he can be harshly blunt at times. Despite largely playing an antagonistic role for the entire series, there is never a moment in which the player fights him, nor does he join the party.
He appears in Golden Sun: Dark Dawn using the alias Arcanus, though little attempt was made to hide his identity. It is heavily implied that he is the father of the playable character Amiti.
Little is known of Alex's history, other than that he lived in the snowy country of Imil for some time with Mia, whom he apprenticed. At some unspecified point in time, he left to pursue his ambition.
Alex joins Saturos and Menardi in their quest to light the four Elemental Lighthouses and break the seal of Alchemy. As an Adept of Mercury, his power is necessary to enter the Mercury Lighthouse. He helps them steal the Elemental Stars, and sees them to success in lighting the Mercury and Venus Lighthouses. After Saturos and Menardi are slain, he informs Agatio and Karst of the situation, but does not trust them to see the mission through to success. Thus, in Golden Sun: The Lost Age, he entrusts the task to Felix and company instead, being impressed with the progress they made. He assists in the lighting of the Jupiter Lighthouse, and sets things in motion for the Mars Lighthouse to be lit. While the party journeys to light the final Lighthouse, Alex secretly makes way to the summit Mt. Aleph. As soon as the seal on Alchemy is broken, this is where the power of the powerful Golden Sun will be formed, and therefore is exactly where he wants to be. He arrives just in time to bask in radiant light beaming down from the heavens. He feels the godlike power surging through his body, and he knows he has succeeded.
In a fit of victory, he tries to test his power in a big way... but nothing happens. The Wise One, guardian of Sol Sanctum, materializes nearby and informs Alex that the power he just gained was not the limitless might he had hoped it to be. In disbelief, Alex tries a show of force against the Wise One, only to be immediately floored by its power. The Wise One explains that it diverted the power of the forming Golden Sun to the Mars Star in Isaac's possession, thus preventing Alex from obtaining its full power. As the world begins to quake from the awakening of the Golden Sun, the Wise One taunts the immobilized Alex, "Mt. Aleph will soon be drawn into the heart of the earth! You must flee or join it forever!" before vanishing. The mountain crumbles away with Alex lying helplessly on the summit.
30 years later in Golden Sun: Dark Dawn, Alex appears again under the alias "Arcanus", working alongside the mysterious Tuaparang. It is unknown how he survived the Golden Sun event, or what he's been up to since, other than apparently fathering the prince of Ayuthay (this is heavily implied but never touched upon). Despite the time that has passed, he retains his youthful appearance due to the effects of the Golden Sun. His intentions are now more enigmatic than ever, as he aids Tuaparang in achieving critical objectives, but betrays them to assist the party during the climax. He is supposedly slain off-screen by the final bosses. But we all know we'll see him again.