Dee Dee is Dexter's annoying older sister. She is often terrorizes his laboratory by dancing around and pushing buttons (usually with her signature phrase, "Ooh! What does THIS button do?"), and is Dexter's biggest problem.
Dee Dee originally was supposed to be the focus of the series, however as the show evolved in pre-production, the main focus switched to Dexter.
Dee Dee was originally voiced by Allison Moore, a college friend of series creator, Genndy Tartakovsky. She voiced the role in the pilot, and the first production season of Dexter's Laboratory in 1996, which spanned 13 episodes. She however, left for New York to pursue a career on Broadway.
Cartoon Network and Genndy Tartakovsky recast Dee Dee with Katheryn Cressida, who took over beginning in the 2nd production season (ep. 14), and continuing the role until the show originally ended production in 1998. She reprised the role for the "Dexter's Ego Trip" movie, which aired one year later.
When the show resumed production in 2001, Genndy Tartakovsky brought back Dee Dee's original voice actress, Allison Moore, to voice the role, as he was interested in making changes to the character. Allison continued to voice Dee Dee for another production season (eps. 53-65), as well as the theatrical "Chicken Scratch" short that was shown before "The Powerpuff Girls Movie". However, Kat was brought back for the next season at the request of Cartoon Network, who felt viewers were most used to her.