Who is my backbone character?
A backbone character is the character in your story who plays the most cohesive role and binds one end of the story to the other. Without the backbone character, the story can't succeed; in fact, when I can't identify my backbone character, I can't even manage to finish a first draft.
Here's the tricky part. A backbone character is usually your main character - but isn't always.
Sometimes a character will appear spontaneously in your head and start telling his or her story, and you'll write it down, then look back and say to yourself, "Gee, that wasn't difficult." Other times, it will be less clear who's holding this story together. Several factors may make identifying the backbone character difficult.
1. You have a story situation in your head, not a person.
If you know more about the world and the danger situation than about a character in the situation, try to zoom in. Figure out who it is who stands to gain most in that situation, by taking risks and possibly losing everything if they don't drive through all the way to the end. Make the situation personal, and you'll have found your backbone character.
2. Your main character isn't doing much "protagging."
Sometimes you can be sure that you know who your main character is, but that person doesn't seem to get anything done. He or she spends a lot of time observing, and when the chance comes to act, typically he or she retreats from doing so. Sometimes it's that you just need to get your protagonist to be more active; other times, you should consider changing who the main character is. Still other times, it's the observer who serves as backbone because even though he or she has fewer opportunities to act, there are still other reasons why that person binds the story together in a way that others can't.
3. You have two or more characters vying for the position of main protagonist.
Choosing who your main character is can be tricky if you've got multiple protagonist points of view. Of the Hero and his Sidekick, which is the backbone character? Well, it depends on what you're trying to do with the story, and how you want its events to be interpreted by readers (as guided by the characters).
4. Your main character is too unreliable to be the primary narrator.
I'm grappling with this one right now. My main character isn't too unreliable yet, but will shortly be descending into madness, and can't serve as a cohesive influence from one end of the story to the other. Therefore, I have to have another person serving as backbone and holding the story together.
5. Your most cohesive character isn't present at the start of the story.
This one is looming in my future. It's really a revision question: I have this story and I always knew who the backbone character was, because without her to pull other elements of the story together, the whole thing would fall apart. The problem with the original draft was that I had the backbone character begin the book - but she isn't the one who starts the conflict. The other characters do that job. It has taken me years to figure out that I need to get the conflict started with the other two active characters, with their goals and stakes, and then bring in my backbone character when her influence can make a measurable change to the trajectory of the main conflict - i.e. when her role as backbone character is most strongly noticeable to readers.
The real challenge in finding a backbone character is to think through what your story is about, and what its core is really made of. The character who has goals and actions and terrible things at stake (the protagonist) may be the same one who endures through the whole thing and keeps the story connected with its core. But it's important to be aware that this isn't always the case. Especially if lots of people have goals and actions and terrible things at stake in your story (which is a good thing!), it's a good idea to think through which character serves as the central organizing influence.
Ask yourself: Who is the character who binds this story together, rather than letting goals and stakes take them off the main conflict in tangential directions? That person is the backbone character, and deserves as much attention as your main character if you want the story to work.