Stephen Hayes#WordWeavers 3 For which characters do you come up with a backstory?
Those for whom other characters in the current story need to know the backstory.
For example in my book "Of Wheels and Witches" 11-year-old Jeffery Davidson goes to spent the school holidays as a paying guest on a farm because his parents are overseas. He meets the farm-owner's niece, 9-year-old Catherine, who is half-Russian, lives in England, and whose parents were killed in a plane crash. He learns this backstory when he is introduced to her.
In a later book in the series this backstory becomes significant when Catherine's grandmother gives her a jewelled cross which is a family heirloom, and becomes a mcguffin in the story.
So the backstory is given on a "need to know" basis -- when either the other characters or the reader need to know it to understand what is going on in the present.