To add to what Mystic said, backstory is also another piece of the "original the character" stereotype. While a character's background is important to you, as the author shaping the story, it's an unmemorable chore to the audience that slows down your story, either animated or written form. You don't ever directly give them that background, it's like dissecting the joke.
In fact, I would say a character is more interesting to the reader when little nibbles of a character's past is fed to the reader, as they become relevant to unfolding events, if they are at all. (I think some trilogies wait as far as the third movie/book to finally reveal something about the protagonist that wasn't known before, assuming it's important to the story.) It provides a small, but complimentary smidgen of interest amid the tension of the story, in wanting to know more as things happen.