Hailing from Greece, Stavros is a very patriotic soldier, caring deeply about his country. He is also very competent, giving vital advice to both his superior and subordinates. He is also a good friend of Esling. When Greece was taken by Stalin's jackboot, Stavros was deeply affected by this Allied defeat.
In the original Red Alert, he becomes vengeful and bitter after Greece falls to the advancing Soviet forces and echoes of war crimes reach the Allied command. Over the course of the latter parts of the campaign, he slowly regains his calmness, as the Allied forces advance on Moscow.
When the Kremlin is finally taken, Stavros enters the city alone and finds Joseph Stalin trapped in a sarcophagus of rubble and being rescued by an Allied rifle infantry squad that arrived just before he did. Stavros, however, ordered the Allied soldiers to move on, the latter being then implied to have understood what was about to happen - the Greek General then stuffed a rag down Stalin's throat before completely burying the dictator and leaving him to die of suffocation.
In Counterstrike, Stavros' experiences during the war are elaborated upon. Under the protection of Tanya Adams he is airdropped into mainland Greece to personally assess the damages in the wake of the Soviet offensive. What he witnessed there was horrible - Soviet grunts execute families in their homes, burn down houses with their inhabitants inside, send dogs on hapless civilians, even use tactical nuclear weapons on retreating Allied forces... for Stravos, this becomes a personal war.
The following is based on the Soviet campaign of Counterstrike and might contradict canon.