Like Vladimir, Stefan was one of the original twelve ruling members of the Romanian coven. Vladimir was the head of a coven of four large-by-normal-vampire-standards, when he joined forces with the other Romanian vampires. His big, successful coven was one of the inspirations for the notion of coexistence to pursue power. During their rule - which lasted nearly 1000 years - the Romanians lived brazenly and decadently. For thousands of miles in every direction, they were known and feared. They ruled over humans and vampires alike, demanding slave labor and copius human sacrifice. Occasionally humans would create armies to try to overthrow what they considered to be beautiful demonic overlords, but all such attacks were futile, resulting in an easy feast for the Romanians. Obedient humans were spared death, if not labor and afew lucky individuals pleased the Romanians enough to earn immortality. Vampires were a well-known fact in the human world when the Volturi began to rise. Their contention that vampires should conceal their existence was scotted at.
When the Volturi defeated the Romanians, most of their coven was destroyed, leaving only Vladimir, Stefan and his mate as survivors. The Volturi copied many aspects of the Romanians' way of life - the formation of a permanent guard, the stationary home, and the eschewing of hunting in favor of prey being rounded up and delivered to them. The difference was that they did all of these things while remaining invisible to humans.
Stefan's mate was killed around 810 AD, during the last of many attempts the Romanians made during the centuries to regroup, repopulate, and topple the Volturi. The Romanians amassed an army of more than 100 new recruits and were confident that the Volturi would fall. This ill-fated assault occurred a decade after the Volturi had acquired Jane and Alec. It was the first time Aro displayed their powers in battle. Stefan and Vladimir were planning to spring a trap for the expected Volturi retreat, and so escaped slaughter when their recruits failed. After this, they made no further attempt to create more vampires for their coven, and decided to disappear until the Volturi weakened or another power presented itself.
The two were constantly on the run and on their guard from the Volturi's agents, but since the two never posed much of a threat anymore, their efforts were not put at their best.
Stefan is noted as cunning, ambitous and vengeful regarding the Volturi's. Though he has a survival instinct, he seems to get it under control when driven by vengeance. He and Vladmir have a tendency of finishing each other's sentences.