Mindy is Mandy's cousin who replaces Dominique and Caitlin after they're written out of the series after "Evil Graduation". After graduating from high school, Mandy was supposed to attend a ski school in Aspen (according to Clover), but transferred to Malibu University to be with Mindy. As such, she becomes a rival to Sam, Alex, and Clover, and to a lesser extent, Britney.
Mindy's basically just a blonde clone of Mandy (has short blond hair, green eyes and very tan skin tone), and has a Mandy's personality, to the point where Sam refers to them as the "Terror Twins" in "Evil Sorority".
One difference is that while Mandy is generally loud and aggressive, Mindy is much more calm and collected, but she is equally gullible, if not more. Mindy also speaks using the Southern American English dialect, showing that she is from the Southern United States.