Another exercise in creating great characters...the old webbing technique is useful. Create a web diagram that shows all the influences on your character, using categories and sub-categories.
For example, a major category may be family, and a subcategory would be grandparents; then describe who they are/were and give them hooks, handles, and descriptors. Think about your character's pets - not only the ones he or she has now, but the ones that have died or run away (always a traumatic experience for a child). Think about school experiences - they are formative. Friends and enemies, teachers, mentors, anyone who's impacted your character needs a small profile, too, especially if they are featured in your story.
A further technique is the interview. Pretend you are talking directly to your character: ask him/her a series of questions as if you were conducting a detailed interview. Follow the thread of the answers you get - you might be surprised. This is especially effective if you feel like you want to add something unexpected to your story. We all have secrets - what's your character's secret? When you find it, don't even include it in the book - it provides a richness to your character without ever being mentioned.
What's his favorite subject in school? What does she hate most in the world? What's his phobia (yes, we all have them - mine are high places and deep water).
In my next post I'll define a character framework.