I believe the key element in any story is something called Transformation. A character starts the story in one place and ends in another. The character can be a person or a place, or even a situation. Without transformation, I believe, we really have no story. I only saw one exception to this maxim, in all the years I've read, and that story (written by someone else) hasn't seen the light of day. It was a study on the writer's part.
Even in stories where the main character doesn't transform, someone or something else should. A prime example of this is The Soul of Caliban, by Emma Lindsey Squier. The dog Caliban, an ugly, vicious brute of an animal, still with odd, unpredictable moments of tenderness, maintained his character throughout the story, even up to the last moment when his owner, Leon Suprenon, shoots him, thinking the dog had just mauled a baby. But it was Suprenon who transforms, too late, as he comes to realize Caliban was defending the baby against a pack of wild wolves.
On a more epic level, Frodo Baggins in The Lord of the Rings changes throughout the span of the story line, coming home from war to see his village changed as well, formerly a pastoral paradise now tainted by pollution and corruption. He is no longer the joyful lad drawn against hi will into the conflict of the story. He is now a man ready to take a forceful stand against the forces that damaged his home.
In my series The NADIA Project, several characters go through their own transformations.
Nadia begins the series in a kind of blissful ignorance. By the end of the first book, she is painfully aware of the truth behind her existence, and has become determined to hold The Pinnacle accountable for their arrogance. She changes in many ways, becoming a little harder in some ways, and more flexible in others as the circumstances around her change, and more facts come to light. These changes come about gradually as the story progresses. But some elements stay constant as well. Her love for Jon is always strong, even though it's tested to its limits in Unalive.
Jon becomes changed from a reckless man haunted by his own mistakes, to one who becomes accountable to an authority closer to him (Dr. Hermsen), to one who finds himself actually seeking guidance from Jimmy, in circumstances he realizes he's totally unequipped to handle.
Jenna is the most fluid of all my characters, and I believe that's why she's the most popular one in the series. She's the bad guy, she's the good guy, she's neither, all rolled into one ball of fury and vengeance. She herself begins the story in blissful ignorance as a Pinnacle enforcer. Then, once she realizes she's being manipulated dishonestly, she begins to take steps to correct the relationship she has with her employers.
Bunny is the dark, snarky loner who finds his match in a dynamic, flamboyant Hushi.
Hushi is the brilliant, vivacious anime princess who realizes moments are too fleeting not to grasp.
Sofi is the crushed, angry survivor who knows she is loved for who she is, and for what she is.
Jimmy and Donna are the two most constant characters in the series, maintain their values and integrity all the way through, even with the hard lessons life has thrown at them.
I think the more you make your characters change, the better your story is, especially if it spans more than one book. Transformation is the heartbeat of the story.
Who is your favorite character in literature, and how do they change?