She is an actress and has been in a ton of commercials. She is described as being tall & thin with huge brown eyes and a "wonderful smile." She insists that the baby-sitters call her by her first name.
Cynthia drives a Jeep Cherokee. The living room of her house is described as having a lot of photos (including the DeWitts' wedding & honeymoon photos, camp-out photos, photos of the Far East and photos of herself in various costumes).
In Book #93, she has an audition at the Friedson\Alper Casting Offices and it is revealed that she does voice-over work at Arctic Air Studios.