Prior to the events of the series, Gauron was already a vicious mercenary. He was once shot in the head by Sousuke in the deserts of Helmajistan, yet survived thanks to a titanium plate protecting his skull. Since this incident, Gauron has been taking on various jobs with different terrorist groups and corrupt political domains. He developed a fixation with Sousuke, relishing the possibility that the two would meet again one day and settle their score.
Full Metal Panic!
Five years following his first encounter with Sousuke, Gauron was hired by a corrupt Soviet official and hatched a daring plot to hijack a plane carrying hundreds of Japanese school students. The purpose for this hijacking was to obtain Kaname Chidori, a girl suspected of being a "Whispered": individuals that contain intimate knowledge of Black Technology within their minds. Gauron captured Kaname and had her subjected to experimentation, but she was rescued by Sousuke. After battling against Sousuke with an Arm Slave equipped with a Lambda Driver, Gauron was defeated and supposedly perished.
Gauron survived his encounter with Sousuke in the Soviet Union and took on another job in Helmajistan, this time transporting a nuclear warhead for an unknown terrorist group. Mithril assembled a group of elite mercenaries, Sousuke among them, to find Gauron and kill him before he reached his destination. However, Gauron was aided by Zaid, a former comrade of Sousuke's, who pilotted a highly sophisticated Arm Slave mecha. While Sousuke was able to defeat Zaid and secure the nuke, all of his teammates were killed and Gauron escaped.
Not long after the Helmajistan debacle, Gauron appeared again commanding a group of extremists that were threatening to destroy a storehouse packed with chemical weapons. Mithril was able to defeat the terrorists and Gauron was captured, and he and his new AS mecha Venom were taken aboard the Tuatha de Danaan, Mithril's state-of-the-art submarine. However, Gauron had planned on being caught and had agents working aboard the de Danaan who set him free. Together, they commandeered the ship and Gauron used the invasive Toy Box program to take control of the ship's AI, Dana.
With the crew of the Tuatha de Danaan held captive and the world's most powerful sub at his command, Gauron was prepared to start a war between Mithril and the United States. However, the maniacal warlord changed his mind after his associates were killed and he thought it would be fun to take the world record for "most expensive suicide", commanding Dana to take the ship as deep as possible so it would be crushed by the tremendous water pressure. Captain Tessa Testarossa, who was a Whispered, was able to disrupt Gauron's scheme by mentally communicating with Kaname, who was on board the de Danaan at the time. Kaname, with Tessa's guidance, took control of the de Danaan by interfacing with the TAROS device hidden within the centre of the ship.
Kaname restored control of Dana to Tessa who commanded her to surface. Gauron was distracted by Tessa's interference and was wounded in the shoulder when Sousuke suddenly forced his way into the command centre and shot at him. Gauron was able to escape and made his way to the ship's hangar where his Venom mech awaited him. Sousuke pursued him and engaged him in his Arbalest. The two mechs could not use their Lambda Drivers in such an enclosed space and so relied on their mono-filament blades.
After the de Danaan surfaced, the hatch for the flight deck opened. Gauron had Sousuke pinned down as an elevator took them outside, the Venom set to self-destruct. Gauron intended to die and take Sousuke with him, but once on the flight deck, after Gauron said " I love you, Kashim" to Sousuke, the AS launch pad was activated and Gauron was catapulted off of the ship. The Venom exploded and Gauron appeared to have perished.
Full Metal Panic!: The Second Raid
Despite the seemingly inescapable circumstances of his defeat, Gauron actually survived his battle aboard the Tuatha de Danaan. The Lambda Driver aboard his Venom AS was capable of shielding him from the self-destruct and he was left adrift and crippled. He was rescued by two former pupils of his, the sisters Xia Yu Fan and Xia Yu Lan. The sisters cared deeply for their "sensei" and sheltered him in Hong Kong.
As his final act of revenge against Sousuke, Gauron ordered the twins to go after Kaname Chidori, using the resources of the terrorist group Amalgam that they also work for. While Sousuke and Mithril are operating against Amalgam forces in Hong Kong, Gauron arranges for Sousuke to meet with him one last time, having Xia Yu Fan plant messages intended for Sousuke all over the city. Sousuke eventually arrives and finds Gauron on his death bed.
Gauron spends his final moments ruthlessly taunting Sousuke, claiming he is weak for succumbing to human emotions during the time he spent protecting Kaname in Japan. He drives Sousuke over the edge as he gloats over Kaname's murder (though this is merely a lie) and the enraged Sousuke finally puts an end to Gauron by repeatedly shooting the crippled psychopath who dies with a maniacal smile on his face.
Gauron also had a bomb planted under his bed set to go off when his heart stopped, his final attempt to kill Sousuke. Sousuke noticed the bomb just in time and leapt out of the window as it went off.
Gauron is a violent sociopath completely devoid of compassion. He cares about nothing and no one, except possibly Sousuke, though that is more of a psychotic obsession. Gauron is a man who doesn't like being told what to do by anyone. He works as a freelance mercenary and is uninterested in the goals and ideologies of the groups or individuals who employ him, though he doesn't seem to care much about profit either. Near the end of the series, Gauron's psychopathic behaviour reaches a head when he decides to kill himself and the entire crew of the Tuatha de Danaan by taking the ship to such depths that it will be crushed by the pressure. His only reason for doing this seems to be merely his own twisted amusement.
Gauron is a brilliant tactician and soldier. He is proficient with a number of firearms, martial arts, and is capable of pilotting an Arm Slave robot. He can even use the Lambda Driver, a piece of Black Technology that can change human thought into physical force.