The part of Grace also had to be played by someone who could pass off the older look, as she needed to 18-19 years old because she was still in school/college. This wasn't difficult to achieve because the actress was 16.
A different girl was originally chosen to play the part as she looked older, but she wasn't blond. We needed the character to be blond because the victim is usually blond. A few examples of the blond girlfriend being used are "Secret Window", "Rear Window" and "American Psycho".
Hitchcock initially introduced this idea in his films. The name "Grace" was taken from "Grace Kelly" who starred in a number of Hitchcock films. The name "Grace" also has religious connotations.