"Victory attained by violence is tantamount to a defeat, for it is momentary." - Mahatma Gandhi.
This character is second in command after the ship's Commanding Officer. He/she is responsible for assisting in the command of the ship during the story and overseeing the SIM in the CO's absence. SIM administrative duties may be delegated to the XO by the CO as needs arise. The First Officer should be the right hand man to the Commanding Officer and capable of stepping in to run the SIM. The qualities that make a good CO, apply for the this position as well. First Officers are selected by the CO of the particular ship.
The XO plays mainly on the bridge, but like the CO, can be moved all throughout the ship and on most of away missions.
Examples from Star Trek are Mr. Spock, Cmdr. Riker, and Cmdr. Chakotay.
This is a command position that is heavily featured in nearly all missions and will see many opportunities for activity.