Companions play a major role in the Fallout series, arguably more so in Fallout: New Vegas than any of the previous titles.  A companion is a permanent follower that you recruit to aid you in certain scenarios- be it combat, picking locks, or hacking computer terminals.  In Fallout 3, companions would only join you if you had a certain karma requirement (evil companions wouldn’t follow you if you had good karma, and vice versa), but this was eliminated in Fallout: New Vegas.  Instead, in New Vegas, each character has their own little nuances about them.  For instance, Boone kills Casesar’s Legion soldiers on site regardless of what you tell him, for his family died at their hands and he wants revenge.  Another example is Rex, a 209 year old cyber-dog suffering from neural degeneration as a result of his old age.  Rex does not like rats, or hats, according to the King, who says his dislike of hats may come from the fact that hat rhymes with rat (gotta love Fallout humor).

In New Vegas, each of these companions also has a separate questline you can complete to “upgrade” them.  When you complete this quest and upgrade your companion, they gain new abilities.  Another difference between the Fallout 3 and New Vegas companions is the fact that you can have two different companions in New Vegas.  This has helped me in many combat scenarios- I have the NCR sniper, Boone, and the cyber-attack-dog, Rex- who unquestionably fight to the death with anything or anyone I decide is an enemy.  Hell, companions are so loyal in Fallout that they won’t even object or fight back if you shoot them in the head a few times.  They’ll just let out a few cuss words (or in Rex’s case, barks) and get back to being your best friend.