Why is Petunia Dursley a flat character?

Answer 1

Flat characters are two dimensional characters who doesn't have any development. Petunia Dursley is a flat character only if you look at individual books, if you look at the series as a whole, she does develop.

Petunia is a minor character so in the first book, we only see her in the first few chapters. Our impression is that she is a selfish and proud woman who cares deeply about how others perceive her. This doesn't change much until the fifth book, when we begin to learn a little more of her backstory and finally in the seventh book were we learn even more about her early life. Prior to this, we only see Petunia acting as she always has and therefore she has no development and is a flat character. This is largely because as a minor character who is a muggle, she doesn't appear in much of the story.

A lot of Petunia's development also happened outside of the books and through Pottermore, which J.K. Rowling used to release new information.