There are several non-playable characters on the field at any given time, with different abilities, AI programming, and alignments. The inclusion of these NPCs function both to make the combat more complex and interesting and to make strategies take both the human and non-human perspective into account.
Marvins
Marvins are the ubiquitous non-playable enemies in Titanfall, and are manufactured by Hammond Robotics as workers. According to the Hammond Robotics site, Marvins are actually called MARVN Automated Assistants.
Spectres
Spectres, unlike their counterpart Marvins, are autonomous "grunt" units for the Militia, and are designed specifically for warfare. Whereas the Marvins are workers first and fighters second, the Spectre class of autonomous units are designed specifically for fighting, making them formidable on the battlefield. Unlike the Marvins however, Spectres are controlled via built-in programming and artificial intelligence as opposed to remote control by a general or commander; while this makes them highly independent and useful for incursions into the line of fire, this removes the ability to control them en masse, which can be a disadvantage in certain situations.