I dont have a lot of advice to say, other than my golden rule: Your query is meant to intrigue and entice, NOT explain the plot of your novel. You do NOT have to tell us every element, you do not have to explain each major character, how they are related, or why they are in the book. You ONLY have to get an agent to think, "Hmm. Now that sounds interesting!" If you have a huge cast of characters, focus on the one with the biggest hook. Reveal enough to tease the agent, and that's it.
So, here is the query letter that worked for me, back in 2006:
Sophisticated New Yorker Jayla Cohen, country bumpkin Samantha Montgomery, and Hollywood hottie Allie Bennet have just one thing in common: their new careers as personal assistants to the stars. But there’s a catch--none of them get to choose the celebrity who they must cater to for the next thirty days.
I’ve been writing for several years now, and at twenty-three years old, I have a good grasp on authentic teen dialogue and voice. I write a quarterly column for SisterDivas, an online women’s magazine.
I look forward to hearing from you, and sharing some of my work. Thank you for your time and consideration.