There's no doubt Pearl is a strong character, but what I want to bring up here is her dependency and obssession over Rose. This was so strong that you could say all she ever did was for Rose, she was never able to do anything without her and during the series we see how lost she feels now that Rose is gone. Change her[Rose's] gender and you get some unfortunate implications.
But while Rose was the first who treated her as more than a "pretty toy", what changed when she continued to act as if Rose was completely above her? Over the course of the show, we see her develop and start being able to be independant and move on, without having to feel as if Rose was everything, while keeping all the respect she had for her and carrying on her wish of protecting Earth.