Tavreynya
Over the course of the story, your uber-bubbly-happy heroine must encounter at least one diehard cynical character and restore his faith in humanity. That's how you know that your heroine is awesome and not, you know, annoying.
(I can see that... "Would you please piss off?" *twitch* "No! Not unless the goodness in your soul finally emerges!" *hits her upside the head and fakes a grin* "Ah, yes! I feel the goodness! Now get lost!"
wink
Extra points if it happens a) at the beginning, so they have a valuable ally throughout the tale, b) in the middle when the heroine is in terrible danger and they're the only one who can save her OMG! c) right in the middle of the final boss fight, when the bad guy is about to strike the finishing blow and the cynical one leaps in and takes the hit!!