The main character of the trilogy is Lyra Belaqua, and her demon Pan. Later on in the trilogy Will comes along.
Demons are people’s souls in the series. They take shapes of animals/birds, and can change, but only while the person is a child. The demons eventually settle as an adult based on the person’s personality. Demons can touch other demons, and humans can touch their own demons, but human’s can never touch another person’s demons. Ever. The reader learns all of this throughout Northern Lights, at different points. You don’t get all the demon info at one point, which I suppose is a good thing.
Do I like this trilogy? Yes, definitely. I do wish, though, that there were more books!
Note: If you like the trilogy even just a tiny bit, you need this book: The Magical Worlds Of Philip Pullman: Inside His Dark Materials. That book is the ultimate inside look into the trilogy. Get it!