Marcia is a squid-like alien, possessing several tentacles and an exposed brain encased in a dome of glass. She is colored aqua green and wears a pink Fake-i-fier around her waist, allowing her to take the form of various Earth objects by spinning the yellow six-spoke symbol to the desired form. Her voice is similar to Betty DeVille from Rugrats or XJ-5 from My Life as a Teenage Robot.
In her human form, Marcia appears as a girl around Timmy's age with red eyes, curly black hair, pink lipstick, and a green bow and pink shirt with the same symbol alien-Marcia has on her Fake-i-fier. If something nice happens to Marcia, for instance getting kissed by a boy, her Fake-i-fier will malfunction and she will lose the ability to remain in human form.