In my Western WIP, it's my MC. He's so weird and creepy and almost endearing. The bad guys are pretty awesome, too. Dirty, filthy, vile things that they are, they're still damn fun to write.
In my new WIP (under 10k), it's the setting. Fall in a small Maine town. Once I get vacation time, I'm planning on heading over to Maine for a week or so, just so it's not painfully obvious it's a Coloradoan writing about a state she's never been to. And the MCs four-year-old daughter is pretty awesome, too.