Lieutenant

The Lieutenant of the Master's Army, also referrd to as Lou Tenant or Lou, is the commander of the Mariposa Military Base and the right hand of the Master

Background
After mutating, the Lieutenant became the model super mutant, being fast, tough, and intelligent. He was the strongest of his batch, and has always been in support of both the Unity and the Master. He firmly beliefs in the superiority of super mutants, as he considers them to be better adapted to the demands of the post-nuclear world.

Appearances
The Lieutenant appears in Fallout and Fallout 2. He is mentioned in the Fallout: New Vegas add-on Dead Money.

Behind the scenes
According to Chris Avellone, the Lieutenant potentially became more intelligent after his FEV treatment.