Se você for pela primeira introdução de Mandark no Episódio "Dexter's Rival" , Mandark é mais inteligente.
When Mandark started going to Huber Elementary School on the first day of the school year, he would end up meeting his future rival Dexter and as soon as they came face to face, Mandark proved that he was the far superior intellect. Upon their first meeting, Mandark proved that he was far smarter than Dexter and even won over the affections of Dexter's beloved teachers. To try and prove that he was still superior, Dexter showed Mandark his laboratory thinking it would mean that he was smarter than him, but then Mandark revealed that he had an even bigger and better laboratory. Mandark then forced Dexter to shut down his lab, or else become Mandark's lackey, and Dexter, unable to let go of his pride, shut down his laboratory.
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Yes, Dexter, I can read your thoughts, and I am smarter than you.
-Mandark ("Rival de Dexter")