Dexter's Laboratory season 5

The series revolves around Dexter (voiced by Christine Cavanaugh in seasons 1–2; Candi Milo in seasons 3–4), a boy-genius who has built a secret laboratory hidden behind a bookcase in his bedroom. The laboratory is filled with highly advanced equipment and access is achieved by speaking various passwords or by activating hidden switches on Dexter's bookshelf (e.g. pulling out a specific book). Though highly intelligent, Dexter often fails at what he has set out to do when he becomes overexcited and makes careless choices. He manages to keep the lab a secret from his clueless, cheerful parents (voiced by Jeff Bennett and Kath Soucie), who never notice any evidence of it. Although he comes from a typical all-American family, Dexter speaks with a thick accent of indeterminate origin. Cavanaugh described it as "an affectation, some kind of accent, we're not quite sure. A small Peter Lorre, but not. Perhaps he's Latino, perhaps he's French. He's a scientist; he knows he needs some kind of accent."[1]  Genndy Tartakovsky explained, "He considers himself a very serious scientist, and all well-known scientists have accents."[2]

Dexter is frequently in conflict with his hyperactive older sister, Dee Dee (voiced by Allison Moore in seasons 1 and 3;Kathryn Cressida in seasons 2 and 4). In spite of Dexter's advanced technology, Dee Dee eludes all manner of security, and once inside her brother's laboratory, she delights in playing haphazardly, often wreaking havoc with his inventions. Though generally dim-witted, Dee Dee occasionally outsmarts her brother and even gives him helpful advice. For his part, Dexter, though annoyed by his intrusive sibling, feels a reluctant affection for her and will come to her defense if she is imperiled.

Dexter's nemesis is a rival boy-genius from his school named Susan "Mandark" Astronomonov[3] [4]  (voiced by Eddie Deezen). Mandark also has his own laboratory, but his schemes are generally evil and designed to gain power while downplaying or destroying Dexter's accomplishments. As the series progresses, Mandark becomes significantly more evil, with his laboratory darker-looking, industrial, and angular in contrast to his original brightly lit lab with rounded features. Because Dexter's inventions are often better than his, Mandark tries to make up for this by stealing Dexter's plans. Mandark's weakness is his love for Dee Dee, though she ignores him and never returns his affections.