Koss is not an Elonian with questionable friends who joined the Sunspears. He's a loyal Elonian who joined the Sunspears, and knows the value of having a network of informants.
Koss can find a way to get along with just about anyone. He gets along with heroes, doing whatever it takes to help the Sunspears protect Elona. He’s courageous enough to say what others are afraid to say, and he'll do what others are afraid to do. He also gets along with the less respectable representatives of Elonian society. For the good of the Order, he's used a combination of aggressive charisma, physical intimidation, and sympathetic morality to build up a network of informants. Of course, some of those contacts aren't entirely trustworthy, which means his information has a price.
Because of a few questionable deals with informers, Koss doesn't entirely get along with his commanding officers. He's managed to talk his way out of being discharged outright, but his extralegal activities have taken a toll on his career. He's skillful enough in combat that he should be an officer by now, but an officer with contacts in the underworld is a risk the Sunspears can't afford to take. Despite that, he knows their investigations need information, so as a loyal Elonian, he'll keep doing what it takes to save his country... no matter how much of a personal sacrifice that might be.
— The Guild Wars Nightfall Manuscripts
Koss is a large, muscular Kournan in the prime of life. He believe the Sunspears can attain anything they set their minds to. A swaggering, almost arrogant figure, Koss has used his charm, imposing physical nature, and flexible morals to build up a large network of contacts.
— Popup window when unlocking Koss
(Note for unfamiliar readers: Kourna is a province of Elona, hence the reference to Koss being both Elonian and Kournan.)