9. Batgirl (Batman & Robin)
Barbara Gordon being Batgirl is no small thing. She’s the daughter of Jim Gordon and hiding that double life from her father is one of the key elements of her life. Moreover, because Batman knows this, he is essentially betraying his staunchest ally’s trust by allowing her to help him.
These traits are a big part of what makes her character engaging and integrated into Batman’s universe. So when Joel Schumacher’s ill-advised Batman & Robin did away with this identity, it lost a lot of what made the character work.
Changing Batgirl into Barbara Wilson, Alfred’s niece, may seem inconsequential to some, but it ruined the character in a lot of ways. She didn't feel like a natural inclusion into Batman's world and her desire to be a crime-fighter just felt forced. None of this was helped by her lack of real development or engaging character traits.
Sadly, this wouldn’t even be Schumacher's worst butchering of a comic book character...