For a reader, the backstory and characteristics of a character should appear to be “just how things are”. But from the writer’s side, it’s a careful process of designing that character’s history, abilities, and skills so that they have exactly what they need to fit into the role they’re required to play.
I had to fine-tune parts of Roz’s backstory carefully to give her enough experience with dressmaking to have developed an ambition to learn more, and enough sewing skills to make up for starting an apprenticeship at such an advanced age (all of sixteen!).