Harry Potter's character could have regressed all the way back up JK Rowling's butthole and her books would still would have had more substance than
Twilight.
And in all fairness
Harry Potter had three more books than
Twilight. Your character's are bound to suffer a little when you're writing purely with the intent of extending a franchise as long as possible for moneys.
Harry Potter had the room to regress, whereas if
Twilight regressed any further, it would become
Scary Godmother.