Here are the current results of the poll:
Not at all. 0 (0%)
Hinted at, but very fully stated. 4 (50%)
Fully formed and present 1 (12.5%)
Fully present, and integrated into gameplay 2 (25%)
Other (please post and explain) 1 (12.5%)
I'll leave the poll running so that people who haven't voted yet may voice their opinions, but in the meantime, I'll probably go with the ideas suggested by Blizzard and F0.
@AliveDrive: Not a bad idea, but I was mainly wondering about a romance plot between the main characters. With NPCs, we don't have to be as careful when it comes to being fully developed characters, so having a comedic-romance side-quest chain is a definite possibility.