Still being relatively new to the world of fiction-writing, I have found one of the most bizarre phenomena is the minor character with illusions of grandeur. He (or she) wants to be a major character. And so far, for me at least, he always wins.
In my early drafts of ROYAL STREET (which appears to be out of the revision phase--yay!), I developed a minor character named Jacob Warin. His main function was just to hang around and be sexy and make major character Alex Warin, his cousin, jealous. Obsessed with "Lost's" Josh Holloway at the time, I gave him dimples and some crafty one-liners.
By then, of course, I was in love with Jacob Warin, and he wasn't going anywhere. Poor Alex.
Same thing happened with the pirate Jean Lafitte. He grew from one brief appearance to start the book, to two scenes, then three, to the grand finale. He damn near took over the second book, RIVER ROAD, so in the third I'm going to send him out of town. I suspect he won't stay there.
My current problem child is named Mirren Kincaid. He's a 6-8", tattoed, curmudgeonly vampire, and in his human life was an Irish mercenary, ca. 1700. He wants my book, and right now I'm still fighting him. I've promised him the starring role in the sequel. So far, he's accepted it. But I'm waiting for him to bare his fangs at any minute.
I'm guessing Eric Northman started out that way in the Sookie Stackhouse series--his role gradually grew as the series progressed from a bit part to comic relief to series sexpot contender to Team Eric, so he'd get my vote for favorite minor character who practically takes over a series. Who's your favorite "once secondary, now major" character?