From Wikipedia

Suzy Creamcheese was a fictionally credited vocalist and character on and in a number of albums by Frank Zappa and the Mothers of Invention. On the album Freak Out! (which features a "letter" from Suzy Creamcheese on the back cover), Suzy Creamcheese was played by Jeannie Vassoir; on Absolutely Free and Mothermania it was Lisa Cohen; and on We're Only in It for the Money, and Uncle Meat it was Pamela Zarubica.

As an indication of the surreal nature of the running Suzy Creamcheese gag in Frank Zappa's work the following from Freak Out! may give a little clue:

Male voice: Suzy?
Female voice: Yes?
Male voice: Suzy Creamcheese?
Female voice: Yes?
Male voice: This is the voice of your conscience, baby... ah, I just want to check one thing out...hope you don't mind.
Female voice: What?
Male voice: Suzy Creamcheese, honey, what's got into you?
Retrieved from "http://en.wikipedia.org/wiki/Suzy_Creamcheese"