Just as every house is built with its owners in mind, there are many attributes to consider when creating story characters, and if you have immersed yourself in the writing world, even a tiny bit, you have undoubtedly come across character fact lists which guide you through choosing everything from hair color, to nationality, to what he or she ate for breakfast yesterday morning. However, while these traits are important for you to know, I believe the three most important elements authors must consider when they develop their characters are genre, motive, and emotion.
First, genre. Especially when it comes to your protagonist. In many ways, genre is almost another word for audience. For example, if you are interested in writing romantic suspense, as I am currently working on, you will recognize your protagonist must be a woman (or a young woman if you are writing for young adults), because the majority of romantic suspense readers are women.
Similarly, if you are writing for a children's magazine such as "the Friend," you will know your protagonist must be a child who is no more than twelve years old (unless there is a compelling reason to make an exception. See my story link, "The Walking Bible.")
The point? Know the kind of story you want to write and the market you want to write it for, then make your protagonist fit.
Second, motive; i.e., what does your character want now, when the story begins, and later, after the inciting incident? What is your character's goal in life? When you can answer those questions, you will not only have grown closer to understanding your character, but you will also have created one of the most important driving forces that will carry your readers through your stories.
Third, emotion. Readers want to feel, to know someone else, even a fictional character, as well, or perhaps even better than they know themselves. They want to connect with their hopes and thoughts. They want to vicariously live another life. And the only way you, the writer, can provide that for them is to create emotion.
Think about it. When you connect with another person, really connect with him, aren't you, in some way, communicating emotionally? And by "emotion," I don't exclusively mean tears. It could also be laughter, trauma, or even the agony of unrequitted love. The important thing is you and your "other person" must feel something. So it is with your readers.