Which of your characters likes the darker side of the seasons and why? Is it for dodgy/evil purposes or does bright light switch them off? What does lurk in the shadows of your fictional world? The great thing with fantasy and science fiction writing is it does leave lots of lovely room for you to invent your own monsters! Okay crime fiction writers can have a lot of fun here too...
What are the monsters in the world you've created and how did they come to get that reputation? Are they really as ugly as their deeds? (I think I would find a good looking monster more scary funnily enough. I would wonder if their good looks distracted people from what they were doing/trying to do).
What is the dark side your main characters are trying to suppress? Do they succeed or, as with My Hyde, does the real them have to come out and make its presence felt?). And who decided your monster character is a monster? Convention? (if so does anyone deny it).