Besides "Underpaid," a new airline pilot or whoever is flying "right seat" is usually referred to as the First Officer unless you are looking for a slang term. That I'll leave to those that have been in that position.
FNG, Fresh Meat, and Greenie all come to mind.
Generally though we save all our good names for the old crusty captains that nobody likes to fly with.
There is no real name for a rookie airline pilot. A first officer could have more flight experience than his captain, due to the first officer's airline going out of business, or someone deciding to go elsewhere. Why do you want a name for a rookie? Just use the same terms for everyone, new or old.
When they first come in the system they are "Green". Usually with low hours and still doing "IOE" Initial Operating Experience. Other then that they are come up with Nicknames pretty fast.
We had a couple of names in our company for the really fresh guys;
F**king New Guy
Norman New Guy
one guy we always called Crash because on his first day he had a car accident and was late.
The older pilots hired on we just called them by name
or technical terms:first officer (co-pilot)
A rookie Pilot is a person who just finished school and just got all his/her licenses. In other words, Any Pilot with a range of about 250 and 500 Hours of Flight Experience. These pilots are usually the ones who are looking for their first job either as a Flight Instructor or as a First Officer at a Small Airline.
Hope this helps!