Bruce Banner's cousin Jennifer Walters was gravely injured by mob violence. Unable to get her to the hospital in time, Bruce gave her an emergency blood transfusion using his gamma-irradiated blood since he had a compatible blood type. As a result she can transform into a female version of the Hulk. She's smaller and less powerful than her cousin, but retains full control while in "hulk" mode.
This series grants her one power that very few heroes possess: the ability to break the "fourth wall" and speak directly to her audience, creators, or anyone else outside of the context of the story. She's completely aware that she is living in a comic book.