The Engineers enhance Saturn's projects, produce and assemble parts, analyze and fix problems with efficiency while their boss is out, serving as assistants and counsellors when he is present. Each engineer seems to have an specialization.
According to Saturn in Nightly Torments, they were created to be master engineers and received an enhanced learning algorithm, which allowed them to solve many issues in the underground world, back in the time when the humans had to evacuate to cities below the Earth's surface. They were not programmed nor ordered to care about the outside world, so they decided to stay underground when humans began leaving.
Saturn heard about their history and searched them for a long time, until a contact informed him about their location and he went to their meet. They were obeying to any order related to their job, even from a Maverick like Saturn, who "tamed" them and brought them to his own hideout. Since then, they have worked for him, oblivious to whatever is happening outside.
The Engineers designs are based in the Engineer class of Team Fortress 2.
Their names are references to the robots present in Isaac Asimov's I, Robot book, which presented the Three Laws of Robotics. They also exaggeratedly inherit some of their characteristics, one of them is that Stephen turned out to be a true robot/reploid and administers the overall data, while Dave is the leader of the six robots.