Lance can be described in one phrase: full of himself. To be fair, this is justified in part. However, more often than not, he praises himself way too much.
Wanting to be a fighter pilot, Lance joined the academy, and after Keith got kicked out, he was indeed granted fighter pilot training. And even met Pidge and Hunk via that.
When not focusing on training, Lance likes to have fun...even when focusing on training Lance likes to have fun. In fact, the only other thing more important than having fun to Lance is impressing females. If any female crosses Lance's path and is even deemed slightly beautiful, Lance is all over them. Metaphorically speaking.
Despite his personality faults, Lance actually is a good pilot, and was the first of the Voltron pilots to get his lion. In combat, Lance is actually a tremendous asset, using his confidence and skill to take out waves of enemy craft. And though he argues with them a lot, he's always there for his teammates, even risking his own life to save them.
Lance may still have some growing up to do before the series is over, but he is a worthy member of Team Voltron. Even if the others don't think so at times.