Usually, you can tell early on how you feel about the main characters in a book you're reading. You know if you identify with them, if you like them, if you want them to succeed.
But you may not know - not at first, at least - if they'll stick with you. You know the ones I mean; the ones you can't get out of your head. You think about them after the book has ended; you're eager to read about them again. Or maybe years later, you still remember their names - and why you liked them so much.
What is it, then, that makes a fictional character memorable?
When I was first created America Miles, my main character in Death on Deadline, I went in a few different directions. I knew she was a journalist, sure, and on that point I never wavered. But somehow, in the first draft, I realized I had created an uber-journalist. She never made a wrong move, or asked the wrong question, or said the wrong thing. But something was wrong. She just didn't seem ...real. So I started over. I recreated her. Several times.
Eventually, I realized my mistake - my fictional creation didn't have any non-fictional flaws. After all, nobody's perfect. Few pretend to be. And for me, it's those human elements, those foibles, that make a character resonate.
Think Jo in Little Women, or maybe Scout in To Kill a Mockingbird. They're outspoken, forthright. But not perfect. They made mistakes, just like you and me. So we root for them. Feel for them. Even cringe for them. And we think more about them.
Or for a more modern take, consider Sookie Stackhouse, the popular telepathic barmaid of Charlaine Harris' Southern Vampire (True Blood) series. Is she memorable? Most definitely. Perfect? Hardly. She talks when it would be better to stay quiet, goes when it's better to stay, and tends to fall for the wrong (undead) men. And like her or not, you still think about her long after you've finished the last page.
It's so tempting, (for me, at least), to bequeath onto our protagonists those better, stronger, faster traits we wish we possessed. But our flaws, our quirks, are what make us interesting, make us who we are.
A few years ago, the USA Network adopted the slogan, "Characters Welcome." Writers more than anyone know exactly what it means.