Sousuke is an approximately 17 year old sergeant within the paramilitary counter-terrorist force, Mithril. He is a member of the SRT (Special Response Team) of the Western Pacific Battle Group "Tuatha de Danaan". His call sign is Urzu-7 and his ID number is B-3128, and operates as team point-man as an Arm Slave specialist.
In addition to being a capable Arm Slave pilot, Sousuke is a specialist in Arm Slave operations and maintenance, improvised explosives, small arms, survival training, anti-armor combat, trap setting and camouflage. Additionally, he is proficient in logistics, demolitions, breach and forced entry, covert intrusion, and hand to hand combat. His background and experience as a guerrilla fighter, and later a mercenary soldier has made him much more sensitive and knowledgeable about tactics and methods more commonly used by guerrillas than government trained regulars.
Sousuke is unfortunately not very good at social interaction off the battlefield. He constantly causes many problems for Kaname Chidori due to his military nature. Even in school he carries a gun with him although it is loaded with non-lethal shells. He is also very bad at blending in with normal members of society due to his nature (i.e. following Kaname with stalker-like aspects and jumping off a train to follow her). That is, he has no readily available "civil" concepts about life, like entertainment or anything recreational and tends to horribly misinterpret the actions of people around him(like considering a guy who dumped his girlfriend a high-level tactician who is to be reckoned with). This is shown throughout the main series in multiple episodes, and the Full Metal Panic? Fumoffu! series focuses mainly on this aspect. This part of his personality is indeed one of the main sources of the series' humor.
His previous name during his guerrilla days was Kashim which was given to him by Majido, a strategist of the guerrilla forces in Helmajistan.
It is assumed that he changed his name to Sagara Sousuke when he left Helmajistan. While in Helmajistan he had a close friend named Zayid.