As a writer especially of Young Adult novels I think it's important to write characters who will make the reader a better person. Or characters who can push through things that a reader might be finding hard. I remember reading books in my tweens and early teens that really just made me feel better, and I think really shaped my personality to a certain degree. This is important, but there's something else certain characters can do for a writer as well, and that is help to to write better.
I'm sure avid readers will be able to pick out certain books by their favorite authors that are just gold: the story and characters mix in the most perfect way and you just can't get enough of it! You'll also probably be able to pick out books by the same author that just didn't do it for you. This is most likely personal preference, but it could also have something to do with the author themselves.
As an author, I will say this: some characters you just fall in love with. All your characters are like your children, and while you love them all, of course, there might be some that you just get along with better than others. I know some books for me are harder to get the prose and the characters right, and take a while for me to get to know them. Other characters just simply talk to me when they want to and say "write this down".
Then there are the characters who speak for themselves and when you write them, you find yourself using words or phrases you never would have thought of but with the voice of the character. One reason I seriously love first person, even though most of my books just don't seem right for it. For me, Anthony Maxwell, my hero detective in my upcoming Steampunk novel, is a character who makes me a better writer. I don't know what it is about him, he just says things and I write them down. It's all him. I can't even imagine how I know how to properly use all those large, interesting words in a sentence--it's all Anthony. Where, in other books, I have to fight with the prose to get it to sound right, Anthony seems to make it effortless. Why is this? I don't really know, one day writers might come up with a word for it. The best I can come up with now is "possession".
It's true that not all your characters are going to speak to you in a way that will make you a better writer, but trust me that when you find them, you'll want to hang on to them. For about two, three, four, five books, or more. And here's a little advice for new writers: Don't speak for the characters, let them speak for themselves. Writing in its purest form should be effortless, and when it is, you'll know that's some of the best writing you have ever done!
So here's a health to our characters who make us better writers! (Raise your coffee cups everyone!)