She is played by Jane Lynch, who is famous for portraying Sue Sylvester on Glee.
Unlike in the Cinderella stories, Dominique is not Mary's stepmother but her legal guardian. Mary's mother was a dancer for Dominique who used to be a successful pop singer. Mary's mother died and Dominique took Mary as her legal guardian and make her a slave.
Dominique treats Mary as a slave and doesn't want somebody nice and good as Mary in the spotlight, even though Mary is very talented and she is excellent in singing and dancing. Dominique used to be a superstar and only made a great income on doing commercials. She found a way to revive her career; she sings a duet with Joey Parker.
Joey would never work with Dominique in a million years. Dominique is grooming her two daughters Britt and Bree Blatt into becoming famous music stars like her and marrying a guy like Joey Parker. Dominique is doing whatever it takes to make sure Mary doesn't get the performing arts school that Mary wanted to go to.
But her plan backfired; not only did Mary get the guy of her dreams Joey Parker, she also got the scholarship go to her Performing Arts School and also got away from Dominique who broke her entire body and Mary won't be bullied or anyone else working for Dominique again.