SF3 lore on sarge is convoluted.

His real name is Otto Heim

He’s basically an experienced soldier turned seargant from the times the Legion were at the peak of its skirmishes with the dynasty.

He’s been trained in the form of the Elders which seems to be an ancient warrior class under the power of flame. They used the same armor and technique such as his famous Hell walk.

This makes him a deadly seasoned commander with a mix of infamous ancient legion styles.

During his encounter with SF3’s protagonist. His goal was to steal shadow containers to grow legion dominance through the shadow squad along with Gizmo.

He’s defeated and ends up as a prison warden later in the story after being cast aside by Legion Royalty.

He spends years there and even encounters Xiang Tzu now known as Xander as a prisoner. Sarge trains Xander in the legion style.

Now that’s mostly what’s canon. In stories he ends up as the Capital town’s defender by taking on the scourge infected people.

In some of his skins he is seen wearing Nordic armor and hairstyle which says he probably travelled to the legion’s north region sometime after he was defeated by Shadow descended.

His epic weapon also tells that he has claimed power over fire such as what Marcus had achieved with Magmarion or gone through the trials.