Mandark

Mandark is Dexter’s arch-nemesis and often attempts to humiliate him amongst the scientific community, or to get a higher grade than him. The two have attempted to work together on several occasions in order to achieve a common goal, but could never do so without fighting.

Personality
Mandark also owns a robot similar to Dexter’s, which he uses to fight him with in an anime-style fashion. Besides the robot, Mandark is depicted as the mirror image of Dexter: he has a huge laboratory and an equally impressive number of inventions.

Mandark is slightly less clever than Dexter. For example, he has an enormous crush on Dee Dee, which very often provides comic relief throughout the series. This is also something Dexter uses to his advantage many times.

When Mandark first appeared on the show, he was shown to be significantly more capable than Dexter, but after the show's revamp, his advantage decreased somewhat, leaving him and Dexter on a more equal ground.

His rivalry with Dexter appears to coincide with his birthname of "Susan" which Dexter laughed at, as well as his secret love of science hidden from his hippie parents. Driven to show himself superior to Dexter, he became his arch-rival, and gave himself a new name; a man's name to reflect his dark brilliance, "Mandark".